What are you talking about? The church says it harms me so much but can't point to the harm. How does it harm me?
Disordered sexual acts (and that's the term I'm going to be using to emphasize that I am talking about sexual
acts here, not necessarily limited to homosexual ones) seperate the the act of sexual intercourse from its purpose, and so glorify the pleasure involved and little else.
Another way you could look at is that sex without purpose is only about giving into pleasures. There is nothing else to the act. Therefore such acts will weaken the virtue of moderation; which is how we resist inappropriate pleasures; which will lead the person to a life in which it is harder to be moral.
On the other side of things you could say that such acts also strengthen the act of lust; which indeed is more or less the glorification of sexual pleaures in and of themselves, this will also make a moral life more difficult. And the vice of lust has its own daughters if strengthened too far.
Though I admit I have difficulty in explaining this particular area. The best that I can say is that a person who does not master his passions will in turn be mastered by them. Glorification of the pleasure of sex as an ends, even if it is a shared ends, certainly does not help in this manner. I think that this is especially true in the modern world, where sex is often encouraged entirely
because people say that we cannot resist the urges toward that end.
But even having written that I don't think that I've said what I wanted to.
There is also the harm done to one's relationship with God, but I don't think you'll let me enter that into the equation so I won't discuss it further.