It's simple biology. It all has to do with physical (and emotional) attraction.
Ok, put it this way. Are you into BDSM? Because I'm not. I do not understand how getting whipped, spanked, abused, etc. is in any way sexually stimulating. Does that take away from the... "legitimacy" of the fetish? No, not at all. Just because I don't understand it doesn't mean it's stupid.
The same thing goes with homosexuality. I've never been attracted to another man, and I'm assuming you haven't either. Does that mean homosexuality is just bullshit and anyone who's gay is just some weirdo? No, of course not. It just means neither you nor I understand it.
Beyond that, you make it seem like humans are the only kind that can be gay.
https://en.wikipedia.org/wiki/List_o...exual_behavior
Ignore the "insect" section. I don't feel like typing out a huge paragraph on why it's more or less wrong, but to put it simply: insects, spiders, and bugs in general, have no brains.