Nope... it's just not what happened. I didn't even know that he had a girlfriend at first. I liked him as a person, sure, but I was not sexually/romantically attracted to him.
It has happened to me several times in my life actually: I meet someone whom I don't find attractive at all (so there goes the "first 5 seconds" silly rule) and whom I later on begin to be attracted to, even after months or years. As I said, I'm sure I'm not unique in this and the fact is, attraction is not that simple. There are no black-and-white "zones" and "ladders".
It's ok to disagree :-).