I would think a lot stems from religious view that merged into society. I live in Florida 10 years ago my home town was country and there was maybe 1 or 2 gay kids in school. Of course they were picked on as such things back then were taboo, now times have changed and it's more accepted by society as a whole but some still carry the views of their parents or older siblings. A majority of people are always going to be standoffish from someone than has a completely different view on life and how it should be. To be honest I didn't want to even associated with someone who was gay or other preference when I was younger but now I have 1 homosexual friend and 3 bi/lesbian friends some people will realize they are still human beings with their own rights and other will always look down upon them. As far as plants and animals are concerned we are still part of the animal kingdom so I would think that it is probably hard wired to a majority of people due to if everyone was gay/trans ECT. their species would most likely die out, but there is a case of a lizard I do believe it indigenous to Australia that is only female and reproduction is stimulation by another lizard rubbing them in the right spot, point being I think regardless life will always find some way to continue, but for all of nature the #1 rule is preservation of your kind so I would think the workings of this world would play a major role in peoples thoughts

Good question, there must be some research on this somewhere. I'd think a combination of factors, but I expect more familial/societal than innate. that's just a layman's observational conclusion, but I've seen attitudes change over the last 60 years, & I know historically that gays were more accepted in some cultures more than others, so that's my take. From the outside, so to speak, so someone gay may have a different viewpoint.

Society sets social norms. Consider the ancient Greeks...homosexuality was accepted by society and therefore individuals accepted it as well. People are generally sheep who will go along with whatever behavior they perceive everyone will accept whether harmful to others or not.

Some of it may just be surface, peer-group hate-mirroring, but when people are homophobic on a deep, emotive level I generally assume they are in conflict with their own inner homoerotic feelings.

Probably a variety of reasons.
Most based on fear, misguided ideas of gender, religion, latent homosexuality manifesting/projecting.
No simple answer really.

a lot of it is cultural--some religious--some peer pressure--some afraid to confront their own sexuality--we have a lot more pressing problems than who people choose to love or have a relationship with

I have no idea dude. What 2 concenting adults do in the privacy of their home is not my business and shouldnt be any others either.

I think it's partially a society thing, but also maybe just a little bit too much thinking about other people's intimate sexual moments. What I would say to someone wondering about that is this: No matter what your sexual preference is, do you actually look at every couple you see (hetero or gay) and imagine them in bed? Not likely - it isn't your business. Let people love in the way that feels right to them and mind your own love life.