Did you drink a lot of alcohol? That'd be why. I have heard of a situation where this one guy asked if the other guy could wait in the other room (for his turn at fucking), because he couldn't get hard. But I don't know if alcohol was involved, it probably was. I heard this from the girl's P.O.V.

First- It's the Alcohol.
Second- It's the Shame.
Choose one or the other.