Actors who stripped down: Kim Cattrall and a handful of nubile co-eds
Why they're fully nude: That's just what happens in locker rooms.
Before there was American Pie there was Porky's (plus Porky's 2 and 3, though it's probably to forget about them), the original "group of male high school students intent on losing their virginity" movie and the one that set the bar for all future scenes that would depict a ladies' locker room/bathroom/etc. as a magical place where all male erotic fantasies come true (i.e. Revenge of the Nerds).
It's in said locker room where all the real nudity goes down, first when Miss Honeywell (Kim Cattrall) gets amorous with a co-worker in the boy's locker room of all places, then as the boys spy on a gaggle of nude ladies, not all of whom are appalled when they realize that they're being watched by a group of randy teens. If you're looking for a deeper meaning, it marked the increase in raunchiness in teen comedies. So, if you love Shannon Elizabeth's "foreign exchange" in American Pie, you have Porky's to thank.