I think the answer might be that if a man is naked when tickled and he is flaccid, the pic looks very obviously faked, and the producers don't want that. Having them in underwear masks this fact. I have seen a few pictures of men tickled naked and it seems 80% of the time they are not erect, which then means nothing to me. Even as a man, if I see a woman tickling a man, I'd rather he be turned on from it.
BTW Maria, if you're looking for new victims, I'm in NJ, too. The name is a hold-over from when I lived near Chicago. And I *do* get erect when tickled!! lol