The concept of marriage is under constant assault in Western culture. Some of it is obvious, such as the attempt to redefine marriage or the attempts to claim that men can be women and vice versa. But these are merely the obvious attempts. Other attempts are much more subtle. In some ways, the obvious attempt distracts more than anything else. They distract people from the more subtle attacks. This is a common way of doing things. It is similar to haggling. You start by setting a higher price and allow someone to talk you down a little bit in order to get the price you are actually seeking while making the person buying the item feel like they have gotten a good deal. To the unwary, it seems like a good deal. They feel like they have gotten the better end of the deal, not knowing that they have actually been taken advantage of. If someone knows the real value of an item though, this kind of thing does not work. Sadly, far too many fall into the category of unwary when it comes to marriage and the things associated with it. Christians need to be aware though. Part of that starts with understanding what is covered in the concept of adultery.
The seventh commandment simply states that we are not to commit adultery. In general, people understand the basics of this. Knowing that adultery covers cheering on one's spouse. But there is much more involved. The catechism gives a good starting point. It asks the question, "What does the seventh commandment teach us?"
That all unchastity is cursed by God. We must therefore detest it from the heart and live chaste and disciplined lives, both within and outside of holy marriage.
The next question gives a further explanation of what is covered by the seventh commandment.
Since we, body and soul, are temples of the Holy Spirit, it is God's will that we keep ourselves pure and holy. Therefore He forbids all unchaste acts, gestures, words, thoughts, desires, and whatever may entice us to unchastity.
Adultery is more than just the act of sex. It is also all the things that would lead up to sex or that might entice someone to sex. In short, even thoughts and words can be adulterous.
In the Sermon on the Mount, the Lord says that if a man looks at a woman with lustful intent, it is the same as if they had done the deed itself. Simply put, adultery is doing anything outside of marriage that should only be done as a part of marriage. Marriage is a covenant between a man, a woman, and the Lord. It entails both a physical connection and an emotional connection. This is in deep contrast with our culture. Even many who claim to be Christians see no issue with participating in things that would fall into the category of adultery. A good example of this is the cultural practice of dating. While a date in and of itself is not adulterous, the way that the relationship is most often played out is. The people involved seek to gain an emotional connection and do things, such as kissing and spending time alone together, oftentimes with no desire to pursue marriage. The root of this issue is that we have lost, or never had, the goal of protecting every part of marriage.
Marriage is more than just the connection between two people. It is more than coming together to make things easier. It is more than sex. It is more than making a family. It is an example of Christ's relationship with the church. It is an exclusive relationship that outweighs any other earthly relationship. The command to commit no adultery protects the marriage relationship in all its facets. The marriage relationship is the foundation of everything other relationships. Marriage is the foundation of the family. A group of families is a community. A group of communities is a country, and so on. God has instituted marriage as the most important earthly relationship and commands that we protect it. This is why the catechism writers talk about the subject in terms of purity and holiness. In a more general sense, Christians are commanded to live lives of holiness. This includes marriage and every other relationship.
The seventh commandment is that we are not to commit adultery. While some may only see this as not having sex outside of marriage, the bible makes it clear throughout that there is much more to it than just sex. Our words and thoughts can be adulterous. The things we watch on our phones or read about in books can be adulterous. Porn is certainly adulterous. But it is even deeper than that. Treating someone who is not your spouse as if they are. This includes most of what our culture does as part of dating. As Christians, we should seek to protect marriage to the best of our ability. Not only our own marriage but the marriage relationships of others. As we interact with friends, we should treat them as if they could be the wife or husband of someone else in the future. As we raise our children, we should teach them to respect marriage and protect both their future marriage and those of everyone they come in contact with. This is likely to cause friction with the culture we live in, but this should be nothing new for Christians. The things of Christ are at odds with the sinfulness of the world we live in. We should seek to glorify God, to fear him and not the world around us.
Soli Deo Gloria
 Lev. 18:30; Eph. 5:3-5.  Jude 22, 23.  I Cor. 7:1-9; I Thess. 4:3-8; Heb. 13:4. Matt. 5:27-29; I Cor. 6:18-20; Eph. 5:3, 4.  I Cor. 15:33; Eph. 5:18.