Yes. I grew up learning that it was wrong. As I made it out of my teens, I pretty much came around quickly.
I do remember that a big part of it for me was that I liked my parents, who taught that it was wrong, and hated the teens I went to high school with who didn't. It felt very culture war-ish to me, though at the time, I didn't realize that this is what I was feeding into. I even knew gay people and had gay friends, and oddly enough, when one confessed to being interested in me, I was like, "naw man, that's cool, I understand, let's be friends anyway and no I'm not scared away." I think now that really, my anti-gay-marriage stuff was really just a culture warrior stance, not that it makes it okay.
Unlike other people, I don't think that the internet helped. Discussion was incendiary on the internet and always will be. It probably pushed me further away from coming around to the right position on the matter.
What changed was basically that, when I thought about it, there was no logical position that one could take against it. I have always been interested in how things work, so I have a range of knowledge that is wide, though only fairly deep in a few places. When we worked on the Amendments in AP Government class, I was like "Well, the Fourteenth Amendment makes logical sense" and it wasn't far from there to "so why should the government restrict gay folks from getting married again?"
I am religious, but I'm definitely not a religious literalist. Almost no one is even if they say otherwise. It was always hard for me to accept that a bunch of ancient Hebrews somehow figured out exactly what specific rules God wanted the whole world to live by, though I do believe in God and come at my belief from a Christian standpoint. Still, if there is a God, capital G, thinking that we could possibly comprehend Him or Her completely is so hubristic as to be laughable. I ended up moving way over past the Episcopalians and into Christian Universalism, and actually it was exposure to Kierkegaard in high school that led me down that path. This was another key to my moving to the proper position on the topic.
I also think that my love of logical fallacies helped. Again, we learned this stuff in high school, and once you learn it, you can't really unlearn it when you see people make really awful arguments. I've never seen anti-gay-marriage people bring any scientific proof that gay marriage destroys civilization or hurts children, but I have seen a whole lot of research to the contrary. Then, the anti-gay argument descends into circular reasoning and faith. I love faith. I think faith is important and, for me at least, precious. As I teach my students, faith has nothing to do with factual evidence or logical arguments. I just couldn't accept the really poorly-constructed arguments toward gay marriage once I began to understand exactly how they were poorly constructed.
It's pretty cool though because all the kids in our family changed the thinking of our parents. We're a close family and talk about stuff all the time, and really, once our parents heard our own positions and why we hold them, they were like, "Yeah, that makes sense, actually." So really, education in general is the key to avoiding prejudice, IMO. Of course, as a teacher, I'd say that for everything!
Sorry for the long post.