I just got back from a (fast!!! On the gofast.) 22 mile out and back on the flat, smooth Rio Grande bike trail (with a 1 mile detour to drop off my daughter's tennis racquet at her nearby school). I wore a pair of good quality, flat-seamed cotton shorts over a pair of slinky and baggy nylon boxers. I generally don't wear these shorts since, though they fit well for other activities, and certainly are well made, they suffer from too high a crotch. This was very noticeable during the first 10 miles or so -- numb nuts, while other, more fully cut shorts, cotton, street, just disappear.
So, all street shorts are not created equal. Personally, I find that the MUSAs I've worn are cut too tight in the thigh and too high in the crotch, but my shorts and knickers are the old models (both Medium) and perhaps the new model is better in this regard.
Some of the best riding shorts are cut from the old style, fully-pleated khakis: plenty of room in thigh, high rise, low crotch seam.