People have a
tendency to think that moving to a more
serious relationship will spoil a
friendship.
Its been my
experience that people who've been good friends for a long time make
excellent couples, largely because there are no
nasty surprises lurking. If he knows you that well, and you know him just as well, then you know what to expect. And,
hopefully, if you knew anything about him that would be a
problem later, you wouldn't have moved to a more serious level in the first place.
In other words, you're all good. :-)