Medusa makes men rock hard.

Anya: What would I have to do?
D'Hoffryn: What you do best. Help wronged women punish evil men.
Anya: Vengeance.
D'Hoffryn: But only to those who deserve it.
Anya: They all deserve it.
D'Hoffryn: Well, that's where I was goin' with that, yeah.