Let brotherly love continue. Do not neglect to show hospitality to strangers, for thereby some have entertained angels unawares. Remember those who are in prison, as though in prison with them, and those who are mistreated, since you also are in the body. Let marriage be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous. Keep your life free from love of money, and be content with what you have, for he has said, "I will never leave you nor forsake you." So we can confidently say, "The Lord is my helper; I will not fear; what can man do to me?" Hebrews 13:1-6
Purity within the bonds of marriage is mentioned next by the author, and this is done in conjunction with honor and judgment.
At the time of this writing, there were many religious people who felt that marriage was a lower calling than celibacy. To them the abstinence from sex was superior in all ways to gratifying one's physical urges. Some men even went so far as to have themselves castrated to prevent them from "sinning." Others simply decided to live as brother and sister in their home, even though they were married. I have met a Christian man who told me he never had sexual relations again with his wife after their sixth year of marriage. And they were married for many decades. This decision was mutual, for they both felt it more spiritual to abstain than to give into the "baser instincts." He felt they were better able to pray and think about the things of God if they weren't thinking about each other in a sexual manner.
Well, that is one extreme. On the other end were those who saw no need to maintain any type of fidelity or chastity in their lives. Alexander the Great had wives, male lovers, and castrated boys at his disposal. Both David and Solomon had multiple wives, even though Scripture prohibited kings from doing so: "He shall not acquire many wives for himself, lest his heart turn away, nor shall he acquire for himself excessive silver and gold" (Deuteronomy 17:17). For many in the first century culture, sexual purity, both within and without the bond of marriage, was "an unreasonable demand to make," said Leon Morris. "It is one of the unrecognized miracles that Christians were able not only to make this demand but to make it stick." 
Marriage is honorable first and foremost because God ordained it (Genesis 2:22-24). Of all the church ordinances, only marriage took place in Eden, in God's presence, in the perfect place God created for man and woman to live. This shows us two things. First, God intended marriage to take place in His presence, and to be entered into by two people that He brought together and made for each other. Because of this I have refused to marry some couples that have wanted me to do so. When a couple is living together, fighting mostly, and not going to church, but then for some strange reason want me to bless their union, I have no hesitancy in saying no. And I have made folks mad because of my stance. The last time I said no, the girl went a long time before she spoke to me again. The couple married anyhow and, unfortunately, it did not last.
The second item this shows us is that the consummation of this union is open to the Lord's approval and takes place within His view. Anytime a man and a woman have sexual intercourse before they are married, which is called fornication, or while married with one who is not their own spouse, which is called adultery, then God's approval is not upon them. God brought Eve to Adam and they were told to be fruitful and increase in number. In other words, have lots of sex and lots of children! So they united themselves in the full view and approval of the Lord who created them, because they were naked and not ashamed, and what God had created for them to enjoy He also declared "good." I have heard Christians tell me that they had sex with someone who was not their spouse, and how badly they felt because they knew they had to leave God outside the bedroom door.
God knows what He created and is not embarrassed by it, and He invites us to enjoy His beautiful gift.
But like all gifts, sex has its place, and that place is within the covenant of marriage. Is this because God is a prude and only wants certain people to enjoy sex? No, it is because God is all wise and knows that if sex takes place outside of marriage, then many people are liable to be hurt, especially a child who may grow up without a father because a husband wasn't thinking of the child or the mother, but only about himself and his "needs." Having sex outside of marriage is like playing a baseball game in the living room of your house. Sooner or later something is going to get broken. Besides, with all the room there is at the ballpark, why would anyone want to cram nine men and a batter in a house? There's a lot more room out on the field, because that is what the ball field was created for. And so it is with sex and marriage. Only the shortsighted think that sex within the ball field of marriage is confining.
Personally, I don't find anything restrictive about making love with my wife and my wife only. I don't find it limiting that I don't worry about AIDS, herpes, or other sexually transmitted diseases. I'm not anxious about sneaking around so that my wife won't find out about my "indiscretions." I've never had the occasion to wonder if a young woman was really eighteen like she said she was. I don't lie awake at night wondering if somebody will call, introduce himself, and tell me that I am his dad. I know for an absolute fact that there are no little Jimmies running around the country, because my wife has always been my only lover. Now that kind of thinking may sound confining and restrictive to those with warped morals, but to me it is a wonderful freedom. Living by God's instructions always is.
As the author moves on to the theme of contentment, it may be interesting to see several passages where sexual impurity and covetousness, or greed, are linked in the New Testament:
"But now I am writing to you not to associate with anyone who bears the name of brother if he is guilty of sexual immorality or greed, or is an idolater, reveler, drunkard, or swindler--not even to eat with such a one" (I Corinthians 5:11).
"They have become callous and have given themselves up to sensuality, greedy to practice every kind of impurity (Ephesians 4:19). But sexual immorality and all impurity or covetousness must not even be named among you, as is proper among saints. For you may be sure of this, that everyone who is sexually immoral or impure, or who is covetous (that is, an idolater) has no inheritance in the kingdom of Christ and God" (Ephesians 5:3,5).
"For this is the will of God, your sanctification: that you abstain from sexual immorality; that each one of you know how to control his own body in holiness and honor, not in the passion of lust like the Gentiles who do not know God; that no one transgress and wrong his brother in this matter" (1 Thessalonians 4:3-6).
We see from these passages that a covetous spirit or attitude can refer to either money or illicit desire (see also 1 Corinthians 6:9,10 and Colossians 3:5). It was therefore a very natural progression for the author to move from purity in our sexual relationships to contentment in relationship to our finances, especially in our love for money. Covetous people covet in all areas of their lives, be it money, sex, power, position, authority, etc., and do so regardless of its effect upon others.
If we put God first in our lives, we will find that coveting what another person has, whether it is his spouse or his bank account, is useless. The first commandment that God gave Moses was to make God first, and to put no other gods before him. The last commandment says, "You shall not covet your neighbor's house; you shall not covet your neighbor's wife, or his male servant, or his female servant, or his ox, or his donkey, or anything that is your neighbor's" (Exodus 20:17). Commandments two through nine all fall quite nicely within those parameters. If we worship only God, and in doing so keep our eyes upon Him and not upon what our neighbor has, we will find no room for other gods, idols, murder, adultery, or stealing.
God is our source and is fully aware of all our needs, and the author quotes Joshua 1:5 and Psalm 118:6,7 to remind us that the Lord is still with us.
We can trust that God is completely reliable in all our interests. Since He has promised to help His children, covetousness is actually opposed to faith. A lack of contentment says that God is not willing or able to supply my needs, and so I must do what I must do to supply myself with my desires. Furthermore, when we covet what we see, and lust after those things that others have, we find that we cannot maintain brotherly love. We cannot look to the interests of others (see Philippians 2:4) while at the same time trying to obtain what our neighbor has, or trying to outdo our neighbor, or trying to work so hard to accumulate "stuff" that in the end we lose not only our relationship with our families, friends and churches, but also our souls: "For what will it profit a man if he gains the whole world and forfeits his life? Or what shall a man give in return for his life?" (Matthew 16:26).
But then, we don't usually covet our actual needs, do we? I live in a house that gives me all I need from a house. It heats in the winter and cools in the summer. It has electricity and indoor plumbing and a roof that keeps water from dripping on my bed. I have a kitchen to cook dinner in and a dining room to feed my family and friends in. I also have a truck to get to work in. It is a 1986 Chevy Silverado, with a unique blue and rust paint job. But it supplies my need for transportation to and from work. I only lack contentment when I see bigger houses and newer trucks and fancier dining rooms and then expend energy going after the bigger and the better models of things I already possess. And when I do that, I am telling God, "Hey, thanks for the house and the truck, but I think I can do better on my own."
But the greedy person is never a happy person. They are ever in search of the bigger and nicer and newer things, always anxious that what they do have is not enough. In reality, the opposite of covetousness is contentment. Jesus said: "Do not be anxious, saying, ‘What shall we eat?' or ‘What shall we drink?' or ‘What shall we wear?' For the Gentiles seek after all these things, and your heavenly Father knows that you need them all. But seek first the kingdom of God and his righteousness, and all these things will be added to you" (Matthew 6:31-33).
On the same theme Paul said: "Now there is great gain in godliness with contentment, for we brought nothing into the world, and we cannot take anything out of the world.
But if we have food and clothing, with these we will be content" (1 Timothy 6:6-8). Paul was the one who also said: "I know what it is to be in need, and I know what it is to have plenty. I have learned the secret of being content in any and every situation, whether well fed or hungry, whether living in plenty or in want. I can do everything through him who gives me strength" (Philippians 4:12-13, NIV). Jesus told us that unbelievers run after food and drink and clothing. Paul reminds us that we brought nothing into the world and we'll take nothing out with us, and since God gives us strength to live in Him no matter what our circumstances, we find contentment in our Creator, not in the things He created.
However, this contentment does not mean we give up working, sit on a hill, and wait for God to supply all our needs. There have been people throughout Church history who have done just that, and Paul said that those people are fit to starve (see 2 Thessalonians 3:10). Our author of this letter to the Hebrew Christians is talking about an intelligent contentment, knowing that God can be trusted and His promises are sure. He will never leave us or forsake us.
This understanding of God's nature not only brings contentment, but also removes all despondency from our lives: "So we take comfort and are encouraged and confidently and boldly say, ‘The Lord is my Helper; I will not be seized with alarm [I will not fear or dread or be terrified]. What can man do to me?" (Amplified Bible). There is no fear of what man will do to God's children, because the love of God in His children's hearts is so large that fear has no room to maneuver, and is therefore cast out (see 1 John 4:18).
So, what can the world do to a Christian that will have any real effect? It can burn our homes, but not touch the place Jesus has prepared for us in heaven. It can take our family, but has no power to remove our place in the family of God. It can fire us from our job, but God is ever powerful in His ways to supply us with a new one. It can kill our body, but then our spirit is free to live forever with God in paradise. And so William Barclay reminds us, " . . . the man of God needs nothing more because he has with him always the presence and the help of God. Nothing that man can give him can improve on that." 
Barclay, William. The Letter to the Hebrews. Philadelphia: The Westminster Press, Revised Edition, 1987.
Morris, Leon. The Expositor's Bible Commentary, Volume 12.Grand Rapids, Michigan: Zondervan, 1981.