It's definitely more brainwashing than anything that makes people think it's repulsive. The same reason a lot of people think masturbation is icky.
It just has to be a social thing.
I remember always liking boys (I'm Bi btw), but that didn't stop society from conditioning me to think of straight relationships as the only normal relationships. I didn't start thinking about having sex with a man until I was in the 11th grade, and started fantasizing about one of my teachers. Up until that point, seeing or even thinking about gay sex just felt "wrong".
However, if it involved two women I thought it was ok because TV characters, guys, girls, and my friends always described lesbian relationships as a sexy/inoffensive/cool thing (which comes with all of it's own issues)
As someone else in the thread put it, straight sex is celebrated in our society.
We can't get away from no matter how hard we try, and unless it involves some weird fetish it'll never be portrayed as negative.
If gay sex is depicted in the media it is usually portrayed as perverted, humorous, dangerous, or just incredibly unusual. Especially when the subjects are two men.
Also, a lot of straight girls don't seem to be "repulsed" by the idea of lesbian sex so I kind of find it hard to believe that it's some natural biological thing.
I guess to answer the OPs question. I don't think one is bigoted for being grossed out by gay sex, but they've definitely got a lot of negative social hangups that they need to reflect on.