Q: What does the seventh commandment forbid?
A: The seventh commandment forbids thinking, saying or doing anything impure.
Ephesians 5:3-4 But sexual immorality and all impurity or covetousness must not even be named among you, as is proper among saints. Let there be no filthiness nor foolish talk nor crude joking, which are out of place, but instead let there be thanksgiving.
Once again, I find Starr Meade's family devotional on the Westminster Shorter Catechism, "Teaching Hearts, Training Minds", particularly helpful this week. She does an excellent job at reminding us of why the seventh commandment forbids what it forbids:
When people break this commandment and commit adultery, they never do it suddenly, with no warning. People break this commandment because they have been thinking about what it would be like to break it. What we do always comes from what we have been thinking. God requires us to be pure in our thinking. That means that the things we put into our minds need to be pure things. Read Philippians 4:8. This verse describes the kinds of things with which we should fill our minds. We should choose the music we listen to, the books we read, and the television shows we watch based on how well this verse describes them. When we first see or hear something that we know is not pure, we do not like it and we think that it is wrong. The more we watch or listen, though, the less it bothers us. Instead of using God's Word to decide these things, we let the shows we watch and the music we hear tell us what is right and what is wrong.
I believe this is a right, and very challenging point to make. Sin, and not just adultery, flourishes when we set our minds and hearts on what is impure. Our outward actions come from what is inside of us. If we desire to live holy, pure lives, we must feed our hearts, minds, and souls with "whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely".