It's just serves no biological purpose. My dick doesn't really magically get hard at the sight of titties either. I generally need some physical activity. There just isn't any difference between wanting to fuck a man or a tree stump. If you want to do it fine but don't pretend it's the same thing as a relationship that can result in the creation of new life.
Discussion
To you there isn't any difference between wanting to fuck a man or a tree stump because you're not fundamentally attracted to either of them. Neither am I. If I had to choose between a beautiful woman and anything else I could put my dick into, it's not even a debate.
Unfortunately some of them feel the same primordial attraction to the same sex, as straight people feel about the opposite sex. It's unfortunate, but that's how it is for them, biological purpose and creating new life have nothing to do with it. If the only way to create new life was to fuck some dude in the ass, it wouldn't change my attraction to women in the slightest, I just wouldn't have children.