The actor-comedian got real about himself doing the deed during an interview with People, published Saturday ... admitting he's less inclined to go for circus-like sex as he's gotten older.

Marlon says he appreciates the simple stuff like "Missionary — it's great. I don't need to swing from ceilings and twist up nothing, because after we're done, I can just go right to sleep."

Basically, sounds like Marlon's not going for anything you might see in "50 Shades of Grey" ... or, "50 Shades of Black" -- his parody movie based on the naughty film series -- for that matter.

FWIW ... Marlon says his outlook on sex has changed a ton as he's gotten older -- adding he appreciates the simpler things in life these days, so maybe he once did go for the more adventurous acts.

Marlon's keeping his life real simple BTW ... saying he's not looking to complicate things with a relationship -- adding he's never had a huge desire to marry. He's got three kids from previous relationships, but he's never tied the knot.

That said, he's not closing the door entirely ... letting people know he's open to it if he finds the right person.