What I have found works with some of my son's obsessions is to just let him indulge in it to some degree, and then say ok, now we are all done with __________ for now. I think the more of a big deal you make of it, the more they are going to want it.
My son is obsessed with car makes and models and churches. But, one of his other obsessions that seems to come and go is bathrooms. Whenever we go places, that is the first thing that he wants to do. This started right after he was potty trained at 4 yrs. old, so I thought it was related to that. So, I was hesitant not to take him right away to the bathroom. Well now, I know that sometimes he has just gone to the bathroom, and we will get somewhere and that is what he wants to do. Yesterday we went to a festival with all types of things for kids to do, and what was he interested in????------The portalets!! ! LOL. But instead of making a big deal, we just took him and let him check it out, and then he was over it.
I think that would be a good idea to ask him why he is interested in ladie's butts, but try not to make a big deal of it. But, if he starts to try to touch ladies when you are out, then you will definitely have to talk to him about that!
Good luck!