Don't know whether it is a cultural phenomen or not, but from the whole context I think Lev. 18 deals with some kind of "sexual nakedness", i.e. if anyone desires to see his/her relatives naked for lust.

Bathing a son, taking a shower with him, going for swimming, etc. has (or should have) a different intention.

~ Little Tree ~

P.S. I have no son so far...