Hi Maria,
Evidently if he's cheating on you and you have felt it necessary to (presumably) do some sneaky checking up to catch him out, then unfortunately there's some deep seated weaknesses in the relationship - I'm really sorry to have to say that.
Without getting too much into the rights and wrongs of it, many men do have diverse and powerful sexual appetites - some, alas, try to satisfy their lusts on the sly.
This is a difficult situation to talk about - not least of all because it's nigh on impossible to have a open, free and frank conversation without it exploding into a blazing row. After all, he knows he's cheating on you and probably would feel guilty and uncomfortable about being caught - and possibly awkward for his sexual tastes being exposed too. The most common psychological defense mechanism is anger.
You are obviously feeling hurt, disappointed and betrayed, which means you're probably not in the best frame of mind for a calm conversation either

Even if you do manage to keep calm, you're probably not going to be able to talk him out of his sexual tastes and, based on the limited context of this thread, I doubt he'd be successful at resisting his desires or satiating them via role play with you (strap-ons or whatever it is that he's looking for...and even that assumes you're willing to entertain such things).
Frankly, the most probable outcome is that this relationship has run its course. Again, I'm sorry to have to say this and I hope you can find happiness in the future. Good luck!