How actors control themselves during steamy scenes with actresses?
@Tetsujin is spot on. I've done a fair bit myself both on camera and on stage, there are so many other critical things going on that it's never been an issue. That being said, I've worked with those for whom it is different - but since it's all quite natural and it only occurs with adults who are focussed professionals, there's never been a problem. If you're having trouble imagining not being turned on constantly, then you're maybe not ready to film intimate scenes yet in your career.
If I were to give advice to an actor embarking on a steamy scene, it would be, focus on playing the character you're playing and their feelings in relation to the other character(s). Like any scene the director yelling cut for the 8th time and the camera guy saying "hair in the gate", an aeroplane passing overhead, a motorbike with a dodgy silencer, a lamp exploding, a makeup artist flitting around with a water spray to simulate sweat - all necessitating faffing around and waiting for things to be sorted, then another take - there really is no way to be anything but 100% focused if you want to finish the shoot. That's the picture.