If 1 Corinthians 6:18 says fornication is the only sin against the body, why isn't gluttony also a sin against your own body?

You make a good point, because how can you have a sin that doesn't involve the body in some way? That would be hard to distinguish, except there's one thing that distinguishes fornication, and that is the idea of one-flesh union. When you fornicate with someone who's not your wife, you become one flesh with her. You can have a one-flesh designation whether you're married or not. When God told Adam to take his wife Eve and become one flesh, that's because when you copulate you're going to be one flesh; that's the definition. But that means if you copulate with someone other than your wife, you're in a one-flesh relationship; you've created another relationship that rivals your marriage, because marriage is the only one that's supposed to have a one-flesh relationship. That rival is always going to be there, because how do you undo a human copulation? You can't. It's always going to be there, and it may even destroy your marriage and take over; that's how strong it can be. And how do you make that one-flesh relationship? By using your body. You've cemented yourself to someone else other than your wife, and it will never go away, always hovering over your marriage. They may forgive you, but they're never going to forget it. That's why Jesus says whoever divorces his wife except for fornication causes her to commit adultery, because fornication is the only competitor against the marriage relationship. In the Catholic Church's understanding you don't get divorced, you get separated, because you're still husband and wife. The one-flesh union you made with somebody else destroyed the relationship, but it doesn't destroy the marriage itself; you remain married but separated from bed and board. That's what Paul means. The Greeks in Corinth thought fornication was next to godliness; in all their temples they were fornicating right at the altars, thinking they were coming closer to their gods. So Paul has to hit this nail on the head: fornication is not like other sins such as gluttony, which you can undo by not eating as much. You can't undo fornication; once you make that one-flesh relationship it's there forever, and it's going to compete and destroy your other relationships. As a Catholic you can go to confession and get rid of the spiritual consequences, but you're not going to get rid of the practical, physical consequences; they'll haunt you until you die.