Add details to the definition of Mandatory Sex Party

This is what I have so far for a definition of Mandatory Sex Party:

A '''Mandatory Sex Party''' (or MSP) is a social gathering where it is customarily accepted that once you walk in the door, you are obligated to have sex.

These parties are often disguised as costume parties, birthday parties or baby showers but it is revealed upon entry into the party that the partygoers have been tricked into attending a mandatory sex party and now must have sex.

I need

Origin of the ritual

Research on Mandatory Sex Parties

Cultural implications of Mandatory Sex Parties

Any other ridiculous details you are willing to provide. Go wild!