Bryan Singer's One of MANY Who Attend Wild Hollywood Parties

The man who accused "X-Men" director Bryan Singer of sexually assaulting him at wild pool parties when he was 15 claimed orgies are common -- and based on what we know ... they are.

Sources familiar with the situation tell TMZ ... there's a weekly party in L.A. that recruits very young men -- advertising 18 and up, but often attended by people with fake IDs who are underage, along with a bunch of very powerful older men in Hollywood.

The parties are usually held every Thursday night at various nightclubs in the Hollywood and West Hollywood areas.

The night of the party, promoters walk around and get the phone numbers of very good looking guys who appear young.

The promoters then text the people who fit the desirable profile and invite them to a weekend party at a private home.

The home parties often have hundreds of guests ... the young guys along with a number of Hollywood heavyweights, including major agents, movie and TV producers and directors and big names in music.

We're told there are plenty of private places for people to go ... but sometimes things happen out in the open.

Toward the end of the evening, a select few are invited to an after-party, and that's where things often swing out of control. We're told the parties often have drugs and alcohol readily available and it's very clear ... sex is the end game.