Firstly, I am so sorry about what happened to your son. I can't imagine how horrified you must have been.
I am a kindergarten teacher, so here is my perspective:
Yes, curiosity is normal. Putting hands down pants and shirts (their own) is normal. Talking about private parts (esp. boys, ime) is normal. I even had a girl a few years back straddling a bar out at recess who yelled, "This feels good!" My aide was pretty horrified, but I didn't even blink an eye. Where I would draw the line is when others are involved. I suppose two kids could come up with something mutually, but what I've seen in my classroom that has been upsetting is when one student pressures another or does something against another's will. For instance, last year I had a lot of issues with one boy who would constantly call girls sexy, hot, baby...you get the picture. He gave a girl a hickey on the playground, talked sexually about a girls' butt when she was bent over, and even went so far as to put his finger down the back of a girl's pants to touch her butt crack, and down the front of another girl's pants to touch her underwear. This kind of thing is not appropriate at all. Apart from the last example, which was supposedly instigated by the boy but mutual (I didn't see it happen), all of the others were harassment. We had several meetings with the parents, who thought we were blowing it out of proportion, but I want to know how they would feel if they were the parents of the girls, or the girls themselves, for that matter. Those girls have a right to feel safe.
I know your son is not doing exactly what I described above, but the pulling a girl's pants down concerns me. For those who think it's not a problem, how would you feel if you were that girl? She should have the right, no matter her age or the age of the instigator, to feel safe and not have that happen to her. It doesn't matter what the reason was that he did it. The result is still the same. The girl could feel humiliated either way. Yes, even at 5.
So that's where I stand on it. I have a boy this year who is overly infatuated with his penis, and to an extent, I think it's normal, but I do have a problem with him chasing other boys around the bathroom with his pants down (he has done this), because he is involving other boys who tell him pretty clearly not to do it.
I am not about shaming sexuality at all, but I am about teaching respect for others and proper boundaries.