My pants aren't spandex or anything, but boxers would be noticeable I'm sure...
What I think you should do, is take a poll. Ask all the girls you train with, what they're wearing underneath there? Then report back. Also count the number of slaps you get.
thongs sometimes- only not w/hardtails-- theyre so low rise that the thong rides up over the pants...too sleazy looking for squats, but ok on the eliptical....haha
I bought a pair of UnderArmour shorts today, the tight-ish ones & they are short-y short shorts & somewhat low rise on me......so I may wear them as my "undies" under shorts, etc so no one sees my pink with green alligator panties! LOL