Female mouth turn-on?

I haven't had any boyfriend that was really focused on my mouth but I don't think I'd find that weird. As long as you are interested in not only her mouth then why would she be worried? She might just be embarrassed that you're staring at her mouth? You could just tell her that each time you look at her mouth you can't but think of kissing her. That certainly wouldn't make her feel unhappy with the situation!