3 situations immediately come to mind:
1. Playing basketball. You guard a guy in the post by pushing your forearm into the guys back, and he tries to back you down by pushing his ass into your crotch / stomach, and you push back with your crotch / stomach. On rebounds, you box out by again pushing your ass into the other guys crotch, and reaching back with your hand touching his hip or stomach. Lots of sweaty skin to skin contact, particularly when playing without shirts.
2. Wrestling. Without a doubt the most homoerotic sport, with true crotch grabs, grinding, even your head being scissor-locked into another guys crotch. Erections on 100% straight guys is not an uncommon thing
3. Chicken fights in the pool. Nothing better than having some guys junk pressed against your neck, your head bumping into his abs, while holding on to his bare thighs.