1) Participation in either penetrating someone's recum, or having someone penatrate yours. . . excessive performance leads to anal leakage on the reciever's part, or bacterial wastes ending up on the performer's member.
2) A good way to get AIDS
All the dudes who like anal sex because they don't have to wear condoms are in for a rude surprise when they get crotch-rot from cornholing a girl who didn't excavate deep enough the last time she wiped her but.