A sex party. Probably like a swingers party, except you don't KNOW that that's what it is until you get there. And, you know, once you get there it's kind of... well... mandatory, to have sex. Because of the mandatoriness of the sex party. And if you don't, then they... you know... rape you. =/

A mandatory sex party is like a normal sex party, except if you don't attend you get arrested. People are told they will be attending a costume party, but what really happens is a wild, out-of-control orgy involving beer, fire, and moving logs onto the road.

"A party where once you walk in the door, you are obligated to have sex. I would think that these parties often disguise themselves as costume parties or birthday parties or baby showers but THEN as soon as they entice people in the door, they are all "guess what this actually is? It's a mandatory sex party, bitch!"

Any party that occurs after some sort of population-reducing apocalypse including, but not limited to, 2nd Ice Age, Zombie apocalypse, Nuclear war, or Donald Trump being elected President of the USA.
In this post-apocalyptic world any gathering of people, such as at a party, will be required to engage in coitus so as to re-populate the world, the only people being exempted are those who are already pregnant.

When someone invites you to a birthday/frat/whatever-other- kind-of-party-you-can-think-of party and it turns out to be a sex party. If you refuse to participate, you will arrested. Hence the "mandatory".

The Mandatory Sex Party is a socially radical minor political party founded in October 2009, in the back woods of Montana. The Party's federal chairpersun is Allie Brosh. To date they have no elected members but momentum behind the MSParty is growing.

The MSParty believes in prosperity through population, coining the slogan, "Breeders are Leaders". In a political climate where many governments are contemplating the challenge of stemming population growth, the MSP takes the stance that population growth will provide the global community with the next generation of scientists, doctors, philosophers and theologians necessary to save the planet. They believe that population suppression risks the prevention of our future leaders being born.

The MSP launched a modest election campaign for November 8, 2011 cantered in Montana, with candidates also campaigning across Wyoming, North and South Dakota. A primary campaign tool being utilised by the MSP in this election race are 'mandatory sex parties'. The MSP, with its many policies supporting family growth, are encouraging supporters to throw pre election mandatory sex parties in the anticipation of having many blossoming families behind their podiums come November 8. Founding member Allie Brosh has...

this is a simple phenomenon, basically just what the words say: it is a party where it is mandatory to have sex -- with allie brosh.

variations have arisen whereby whatever girl or guy that is named becomes (he or she) with whom sex must be had if you want the free drinks and food etc