There are people other than clergy who find themselves in a position of restraining their sexuality so as not to commit sin. Widows, widowers, single people.
The other reason is that in the Christian next life, there is no sex, so you can get a head start here. Regular enjoyment of sex, while normal if one is doing it with one's lawful spouse, *can* alter your spiritual priorities.
In an orderly society, sexual discipline really works for the good of all, whereas we now see in our society the downside of sexual laxity.
If sexuality weren't regulated to some extent, society as we know it would completely collapse. In other words, you can't just always do it because it feels good without severe consequences. Better to get a handle on it while you can and if you can.