It's hard to say. In my experience, ENFP's like a lot of attention and they need to feel appreciated for everything they are and do. I could see one straying if not given enough of these two things.
I know an ENFP male (test confirmed) that has been obsessed with me for years now and he shows no signs of giving up. He believes in fate and love at first sight and thinks we are meant to be together. What I think is really going on is that he is confusing love with lust/attraction.
I won't give him a chance which makes him even more persistent. I do feel that he likes the chase/challenge. He can't understand why I don't want him. He is such a great catch: successful, brilliant, and good looking (his words). But he left out one thing:he is MARRIED. I would have dated him if that weren't the case.
So yes, they can stray if their specific needs aren't being met. From what I gather, his wife does not say thank you enough for all of the wonderful things he does, she doesn't mentally stimulate him, and she isn't affectionate enough. Things like that. Not that I am any of those things for him but we do have great conversation.
I think if you want to keep them interested, you should provide mental stimulation and play hard to get (not what I'm doing but you get the point). And be attractive. If you cover all of their needs, then I do not believe they will stray.
But not every ENFP is alike.
