Lots of animals also engage in incest, though. And we all know what happens with incest.
Other nasty yet natural things animals do :
Tongue their balls
Tongue their ass
Luckily, we don't have to because we have showers, but eh, go ahead and throw the " natural " argument out the window. Incest is very common especially for domestic breeds of dogs and cats in the USA, and homosexual activity probably is too, so nothing stops homosexuality from being natural; in that logic, just as natural as incest.