When I first fell into HP fandom around 2001/2002, I had no idea slash existed, nevermind Snarry. And when I found out about Snarry, I was originally entirely grossed out! I wouldn't go near it, and as far as I was concerned, Harry/Draco was my OTP and would be forever. Of course, then Book 6 was released.
Book 6 was a game changer for me. I had never been particularly interested in Snape as a character, but I became completely obsessed with him after that. I was utterly convinced that he'd only killed Dumbledore on Dumbledore's own orders and was still working for the Light, if you will. I wanted to get everyone's take on him, and the easiest way to do that was through fanfic. I bit the bullet and decided I'd try reading Snarry, so I wandered over to Switchknife's Slash Recs to find something that might fit my tastes. I can't remember which one I read first, but it was a combination of A Convenient Marriage, Beech Hall, and Hall of the Mountain Kings that sealed the deal - I just started reading and couldn't stop. I didn't read anything else.
After many, many months of wondering about it, I was forced to concede that I no longer had an OTP - I had two. x.x Which, of course, is a bit of a contradiction.