1. You broke your penis.
2. Your boner won't cooperate after a few drinks.
3. You notice blood in your semen.
4. You have blue balls.
5. You keep going and going and going and can't ejaculate.
6. You orgasm really really quickly and you're upset about it.
7. Your penis is really bent and it hurts.
8. You've got jock itch.
This is actually called tinia cruris, and it's a fungal infection on your groin or genitals. You might notice some itchiness or redness in the creases by your thighs and butt, and in unfortunate cases, it can extend to your scrotum or genitals. It's caused by heat, sweat, and skin rubbing together, so wearing breathable fabrics and loose-fitting clothing can help. And if you sweat a lot (especially in the ball region), Morgentaler suggests using a talcum powder to prevent jock itch. Make sure to also wash and dry your genitals properly to prevent it. If you've already got it, there are OTC anti-fungal creams that can help, or you can go to your doctor for a prescription-strength one.