most of this is really old photography, some of it has some frontal nudity so be wanred...
the poses and lighting are really good and there's an ok variation in body type...
another one with some ok refs. there's a lot of classic drawing int here too that lookinsanely photo-realistic:
and yet another, most of it contains frontal nudity, it's mostly just muscly people. the poses are ok i guess. i barely use this site.