I always wondered the same thing actually. Like there is a difference but not much. I would probably say underwear is made to support boobies and be snug downstairs for ladies and bikini is more to get wet. Muffins are lots more dense than cupcakes and no frosting.
If you are making the comparison that bikinis are OK at the beach and underwear are weird in town..yeah..pretty dumb..btw..I never care if anyone sees me in my briefs...although..I am not an exhibitionist..kind of a ham tho../: ..I am ignorant..what is the difference between muffins & cupcakes?