Sex scenes need to be handled with a lot of care from everyone on the production staff. If you're asking actors to be intimate, you need to set boundaries, have long conversations, and make sure everyone is comfortable. 

David Thackeray is an intimacy coordinator for places like Netflix, Warner Bros., Apple TV, BBC, and HBO. He says these scenes are some of the hardest for every production. And that's because not only do you have to make sure everyone is on the same page and into it, but at times, people can become too into it, get aroused, and need a break.

He says actors "are going through the physical and the mind, so yeah, it happens."

How to approach intimacy on sets

There's no perfect way to shoot one of these scenes. The primary goal is just to find comfort for the people involved. That might mean clearing the set, and also talking with the crew about what's happening on the scene, so no one is surprised and so you don't have to do too many takes.  

Still, you might have to cut if an actor's modesty garment shifts, the choreography is off, or if someone gets too aroused.

In those cases, Thackeray said, "Just making it really clear that's normal and that the worst thing you can do is carry on. So we call that a time-out. Give them five minutes then I come in and check-in. […] Then we come back into it when they're ready."

So yes, it happens, and if it does, send them off to calm down, situate, and then reset. It's not too complicated, but I think worth noting as a real thing that happens and has to be dealt with as it occurs. 

Let us know if you have experience with this in the comments.