Conditioning. Pretty much everything comes down to conditioning. Anything sexual, especially, is easy to associate with other stimuli.
You've probably been subject to subtle and overt conditioning all your life to find homosexuality repugnant. Hearing someone being bullied with slurs like "faggot", for example, creates a negative impression.
Personally, I don't have much of a problem with anything like that but it's taken a lot of work watching porn to reach a point of apathy. Like, the first time I saw a ... lets call it a "double" ... I was like, "omg, they're touching." Now, I don't even care.
One thing I can't stand, though, is hyper-feminine gay guys. I mean, I'm fine with them being homosexual but they really annoy me when they act like caricatures of women. Of course, women acting like that would also annoy me so it's probably less about homosexuality than simply about people being annoying.
