How do they really make sex scenes look so believable in the movies?

Acting!

Believable? Does anyone actually think they look like the movie stars do when they're going at it? I think we're just conditioned to movie sex, like movies gun shots and movie fistfights.

That said, there was an interesting dvd extra once discussing horseback riding in a movie where the actors were discussing how you make it look realistic when you're not on a galloping horse but rather a barrel on the side of a pickup truck. They universally said that, embarrassingly, the effective look was achieved by simulating sex on the barrel. So same principles I guess.

unapologetic_mocker's got it right- movie sex scenes aren't that much like real people having sex any more than movie dialogue is like how people really talk or fights are how people really fight. It's just what we've come to expect from the movies.

Speak for yourself! I always make sure that we're both covered by sheets during sex, just as in the movies. Likewise I never leave the surface of the bed without first donning my underpants, and the missus either puts on bra and panties or wears a whole bed sheet to the bathroom. It makes us feel like movie stars!

If you think movie style sex is hard, you should try to urinate, fart and defecate without making any sound...

Yeah, I have a whole closet full of L-shaped sheets. Post nookie we can lie with her chest covered but my rippling abs and pecs exposed for nobody in particular.

 But of course! No doubt yours (like mine) are covered with Teflon, so that any bodily fluids won't show...

Oh god. make the mental image go away.

And to the OP, I imagine it's not difficult to jam one's genitals against an attractive actress and appear titillated.