Why do girls think it's 'weird' when a guy doesn't ask for sex and wants to take things slow?

I think she might be more surprised that you haven't, a lot of us women are lead to believe that most guys are sex hounds so when we find a decent guy, we wonder why they don't do something sexual. I don't think you're letting her down, most likely she respects that you respect her and that you are taking your time. Just do whatever you are comfortable with and work some more intimate things in one at a time. I'm sure that if she wants it bad enough she may even make a move on you first.