I know, old thread, just wanted to add my 2 cents worth. I ride a midsize cruiser with a Mustang touring seat. What I wear underneath depends on weather, and how far I'm planning to ride that day, but it could be anything from commando, bicycle compression shorts, UnderArmor compression shorts, jockstrap (no cup), boxer briefs, or mesh boxer shorts. I've tried mesh jockstraps, but the ones I tried had a really loose weave, and I didn't like the feeling of my scrotum being pushed through the holes.
When its really hot out, the feeling of the wind through mesh boxers under my mesh riding pants is incredible.