Do they really have sex in movies?

It's all pretend.

They do in the pornographic ones (so I've been told).

I heard it was the real deal in Deliverance.

if there is liquid shooting, it's likely real.

It has happened. However, usually not

In mainstream movies, the sex scenes are simulated. Sometimes, the actor and actress are filmed separately, and aren't even on set at the same time.

The guy received a real blowey in the brown bunny.

I know of two movies in which the actors claimed the sex was real--and not consensual. All the others are actors doing what they do, pretending to have sex for the camera. Most of the time they're wearing something that covers their genitalia, so they are not on display to fellow cast members and the many crew members it takes to film a scene.

No - of course not. Not in real movies.

They're actors. Not porn performers or prostitutes.

When there's a sex scene there are dozens of people all around the actors, so it's not actually even slightly sexy, erotic, exciting, etc. They also wear skin-coloured thongs most of the time.

They are acting. Most movies aren't porn.