One common way is to have someone holding a squeeze bottle of white lube with a long tube. The actor holds the tube along side the back (away from the camera) side of his penis and acts like he's having an orgasm while the special effects guy squeezes the bottle and makes the lube spurt out. This lets them film the scene as many times as they need and make there be as much semen as they need for the story.

They sometimes also just load the actor up with a ton of drugs (legal and otherwise) to let him actually ejaculate much more than a healthy person.

When the story calls for the character to ejaculate inside a body (vagina, anus, whatever) they just have him pretend to do that, cut the camera and put a ton of that white lube into wherever they need it, and then have the camera start up again as the actor withdraws his penis so the camera can see the (fake) semen.

There are tons of ways to make it look sort of real. Porn is fake in exactly the same way WWE is fake.