It's unnatural in the sense that it doesn't have as aim to recreate, which is pretty much the most important goal in nature. I see little difference between homosexuality and incest, pedophilia or bestiality really. I'm not saying it's morally wrong though.
Now, I just know some of you might say "Hey, what about impotent people." Well, it's the natural attraction that is unnatural. Being attracted to a girl (as a man), whether or not she can have children, is natural (since your brain and hormones don't know she can't have kids). Being attracted to a person of the same sex, or to an animal, is generally, in biological development, strange (to say the least).
Homosexuality at its roots is a biological error. It's not natural. And hold your "Hey, but animals have homosexualtiy too." arguments away. It's still an exception that has no benefit for the well-being and procreation of the species, and thus, isn't a "good" thing.