6yr old DS obsessed with private parts

[CEDP]

My nearly 6-yr old son is obsessed with his private parts - and increasingly those of his friends. Showing is turning into touching and I am concerned that things could all go too far. Is this normal? Advice please!

Both my DSs aged 4 and 5 are same; we had to have strict rule in bathtub that it was ok to pull on your own parts but not on anybody else's - particularly when daddy was also in the bath .
I think it is natural curiosity and nothing to worry about. We do discuss that there is a time and a place for everything ie not in public.
A friend whose son is 14 only said when I recently expressed being fed up with DSs obsession (and toilet humour): "It does not get any better when they get older!"
DH is now 38 and I suppose my friend is right...