I don't think it's possible. You can't just shut it off, but you can control yourself. Casual sex in all ages is pretty normal now, but I think that's because everyone likes to use the excuse 'it just happened' 'I couldn't control myself.' Or now you have that whole silly YOLO thing going around.
I've talked with my boyfriend about lust, and you really can't help but find someone attractive. I'm straight, but there are girls out there that I'm \like, wow, she's hot. And I'm sure sometimes everyone thinks what sex might be like with a person they find attractive, but doesn't mean it has to be acted on.
Self control is part of what we think makes the human race so inferior, but I don't think when it comes to lust especially, that it's well practiced.
And actually it's probably true for some people to lust more when they aren't being sexual pleased in their relationship. (Not that it is ever okay to cheat, I'm biased on this matter.) But for a lot of people sex is a vital part in a relationship and if it's something their partner isn't sharing with them, they may entertain some other thoughts. Whether they act on those or not.