It was mostly due to rude comments from people. They didn't win their case with me by rude insults.
There was also my personal experience dealing with those who were either self proclaiming gays or those in the closet who secretly approved of homosexuality. The biggest thing was a so-called friend, who tried to manipulate me. He was married, went to church (found out later he didn't believe), and pretended to be my friend. He was not interested in platonic friendship, and he tried every way to seduce me. I didn't believe he was gay, and even when he told me, I thought he was just joking. When I didn't catch on to what he was doing and reply in the manner he was hoping for--he ditched me as a friend. It was the worst time of my life. He didn't care about me personally as a friend, but was only trying to get his rocks off. Although, it wasn't quite repeated to this extent, every gay person I ever tried to befriend would bail when they realized I was only looking for friends. They were always pushing for sex, and as soon as I heard the stereotypical question, "you a top or bottom", I replied neither, just looking for friends--click, end of conversation.
Tha pretty much sums it up.