I don't fully know the answer to this (since I'm not gay), but let me hazard a guess: what we find sexually invigorating doesn't generally involve children unless someone is a pedophile. I don't have children yet, although I imagine I will at some point in the future, but when I see a naked lady, my immediate thought is not "I want to put a baby in that." The desire to be a parent and the desire to achieve orgasm have basically nothing to do with each other except that one sometimes leads to the other.
So are you suggesting that homosexuality only exists for sexual pleasure?
I figured that humans want to pass their genes on, homosexuals can't do that, so they pass other things on (likes, dislikes, hisorty, morals, etc) but that innate desire for kids never leave, so they adopt.
But... if the innate desire for the opposite sex leaves (or doesn't exists), so should the desire to have a family.