:) I agree 100%. Very normal and not bad. Teach her to do it in her room, that it's not appropriate to do in public. She simply does it because it feels good, she has no other intention. It's like you getting a back rub.
Sexualized behavior in children varies in frequency, type and intensity. But the behavior in an of itself is not abnormal. Many children this age engage in behaviors of this kind. At her age, she can begin to understand concepts of private vs. public. That should be the focus, just as if she was (you'll pardon me) picking her nose in public. Also, sexual behavior in the form you described can serve a self soothing function. She may be feeling anxious in certain situations and be more inclined to act that way. If so, perhaps you can find alternate behaviors that she can do to learn to soothe herself (e.g., deep breathing, jumping in place, etc). Talk to her and think about setting up a signal that reminds her about no touching in public places to minimize embarrassment and decrease the behavior. Try to think - how would I handle this if it wasn't about a sexual behavior but some other behavior. Once you add sex to the mix I think we all become discombobulated.