Because it is excellent exercise and they are open-minded guys?

Because Sam's a perv?

Because the Brain's a perv?

Even though it makes good visuals

and, really, need we say more?

:)

Hmmmm. And I should probably blame someone besides fictional characters and myself at this point, so I'll also generously include green_grrl, who blithely mentioned that she has some sort of sarong fic laying around but has never produced it and it's been eating away at my subconscious ever since?