Well, it depends on how hard you are working out, and how much you are sweating etc. Cutoff jeans and cotton tightie whities underneath are a nice recipe for a rash in a place you don't want a rash. Also you don't want something that will constrict your range of motion if doing stuff like squats or rowing etc. Workout clothes need to be functional for your workout, but they don't need to be super cute.