I used to think homosexuality was a choice and had a pretty solid argument when I was younger, as far as my opinion back then was concerned (now I wonder how I thought it was intelligent). But I was never hateful or "against" homosexuality, I just thought it was a different mindset and it never changed my opinion on a person's merit.
Why it changed was because I had one of my little thinking sessions when I was 15 and realized; "wait a goddamn second. I didn't choose to like chicks only, I just always did... MY GOD I'M AN IDIOT"