I have no idea, I just wanted to say something stupid.....well seeing men having homosexual sex with each other seems wrong, even anatomy doesn't agree with it.
Physically, homosexuality seems wrong. If you go down to chemistry though particularly in the area of the brain related to sex and sexuality, you will see some chemical differences.
As for why homophobia exists? Generally, people say it is unnatural. Some people use religion, but most people are selective about the Bible anyways (especially with the Old Testament, which is actually incoherent on homosexuality due to having a few quotes saying it is wrong, but having a few stories where homosexuality was present). Really, it comes down to the people. There argument? Generally it is unnatural, but there is a flaw:
Homosexuality is phenetic
Anything that is phenetic is deemed natural
Homosexuality is natural.
It is worth noting that the New Testament quotes generally refer to sodomites, and only once refers to homosexuality, using the reasoning that is is unnatural. That cannot be true if homosexuality is phenetic.
If you want to argue that homosexuality is a direct obvious choice that involve you doing actions (as opposed to actions be done upon you) with predictable outcomes EVEN FOR A CHILD as opposed to being at the very least phenetic, go ahead. I doubt you will get far though. There is some strong inductive reasoning, such as why would so many homosexuals go into such a lifestyle if it were a choice and they could make instantaneously? Why is reperative therapy such a failure and almost never has a life-long result (and the few that do get life-long results doesn't justify the problems that come out of it). Go on and on. It is easier to admit that homosexuality is phenetic than to argue the case that homosexuality is a direct choice that involve obvious choices that involve you doing actions with predictable outcomes even for a child.
And yes, the whole "Direct choice..." seems extraneous, but if one of those were not there the entire argument of homosexuality not being phenetic would fall apart.
Really, homophobia exists because people don't want to accept that it is natural, they don't like the idea, they want to hang on to tradition or religion, et cetera, et cetera. Simple put, people don't want to change.