Has he asked you questions about sex and gender differences, reproduction, where did I come from, and of that sort of stuff? Can he articulate his motivations for pulling his pants down and wanting little girls to do the same?
In response to your question, he has said, "I am curious!" as his explanation for why this has happened. He often laughs about it ("We saw each others' penises!") and acts like it's a funny thing. Honestly, I don't think there's anything sexual about it, in the sense of wanting to be involved in a sexual or arousing act. My son isn't even sexual with himself as far as touching himself. It's not like he's obsessed with sexuality or stimulation. All of his discussion about "private areas" revolves around joking about it and laughing about it and shaking his booty around, like other kids do. He only showed shame about it when he had been told a few times NOT to be pulling his pants down and still did it, yet he didn't keep it a secret from me. I think if he hadn't been told a few times not to do this, he would've just been laughing and saying "I saw her private area!!!"
Now you could say that the fact he's done this when he's been told not to shows a problem or obsession, but like others mentioned (e.g. "I tell my son not to hit his brother, but he still does it anyway"), I also tell him not to do a lot of things that he still does! Isn't that part of being five?
There's also been a lot of talk at school amongst the other boys with them saying things to each other like "kiss my butt" and "smell my butt" (clothed, putting their butts in each others' faces and laughing). It's very clear to me that in his mind and the other kids who are saying these things that this is 1) funny, and 2) disgusting. It seems like very typical boy stuff to me. None of this has been initiated by my son, but by the other kids at school.
I agree with what some of the other posters have said, that just because he's had other issues, I personally don't think that this means that ALL of his behavior is abnormal. I did talk about this in detail with his therapist today (when my son wasn't there), and she said she thought this was VERY normal for this age, even asking another child to touch his butt hole, which my son said didn't happen. My son's therapist said fascination with genitals and even anuses is very normal for this age, and that just because he asked someone to touch that area doesn't mean it's sexual. She did say that since he seems to be having issues with keeping his hands to himself and his pants on (even though she said what he's doing is completely normal, even if touching was involved, which it hasn't been yet), that he shouldn't be taken to an play area where I can't see him, which is a conclusion I have already reached.
Honestly, I believe him, as I have seen this other little girl (my friend's daughter) blatantly lie to her mom MANY times. I have witnessed incidents where she wasn't just misinterpreting what happened, but flat out lying about it, My son has only lied to me once or twice. He rarely lies. So this makes it very hard for me to completely believe what my friend's daughter said, since she seems to have a significant issue with lying.
Now, all of this said, it's not like I am just saying, "This is all completely fine and normal," but I don't think he's a few years away from becoming a sexual predator, as some seem to have suggested. I think his behavior seems like normal curiosity and silliness. BUT I am still going to make sure that aside from school (which I can't monitor, and that bugs me) I won't put him into situations where he might be tempted to continue this behavior. I will have to make sure I can always see what he's doing. This means we can no longer go to our favorite place for him to play, which will be a huge loss for us (especially for me as a single mom, as I could sit there and work on my laptop while he played), but I don't want this to become a compulsive behavior. This means I won't be able to take him to the gym childcare center so I can work out, as they don't supervise the children there as closely as they could, and there are some play structures there that would also allow him to be hidden. I am not sure if he'd try this with a child he just met, as this has only happened with kids he knows, but I want to be careful. I don't think he has a serious problem, but I don't want anything to develop into a serious problem, so I am going to remove him from the opportunity to be involved in these activities, except for school, which I don't have control over. I have talked about this with his teacher and the principal, but there are times when he might be alone with other kids, like in the bathroom.
I have ordered a bunch of books about the human body and keeping your private areas private also, so hopefully that will help with his curiosity.
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