I would like to suggest that something very normal might be going on. It is my understanding that some men are simply genetically constructed so that they must have variety in order to perform.
"I have to get some strange or I can't make love to my wife."
"The next time I was with my wife, I didn't have any problems getting an erection and it felt like I was making love to a new woman. Her vagina felt different. It was like we were on our honeymoon again."
"I like the idea of monogamy and I aspire to it. I'm just not very good at it."
I am a sexologist. The statements above are the kinds of things my clients say to me, when they are concerned about their need to see professional sex workers, in order to "make it work" at home.
The reason prostitution is the oldest profession, and it has survived for so long, despite the stigma, is because men need to have sex with a variety of women in order to feel normal, and keep their marriages together. Some men are apparently simply hardwired in such a way, so that they are in the very strange paradoxical predicament of having to be unfaithful, or else the wife will think he doesn't love her, that something is wrong with him, fear he must be having an affair with someone else. It is a fluke in nature, a perverse tweak of the human sexual condition, and ironic result of natural selection. Polyamorous men apparently passed on a lot of genes to future generations.
How large are his testicles? One hypothesis is that men with large testicles are more likely to need some strange. Indeed, in species of mammals that mate monogamously, the males have smaller balls.
What if all he needed to rejuvenate his passion for you was to spend an hour or two with an escort? Could you handle it? What if she taught him things that made him more confident and sensitive to your needs and he came home to you a much better lover, for having spent time with her?
If that's what is going on, don't worry. It's just a maturation process, a phase he is going through. Men need variety when they are younger, but they get bored with it when they reach middle age and start wishing for something deeper, more meaningful and spiritual... about the same age when women hit their peak and start feeling the urge for some strange...
Nature apparently enjoys practical jokes.
If he doesn't have large testicles, perhaps there is a psychological component. Men need their freedom and autonomy. When they start feeling too enmeshed with a woman, too secure in a relationship, it can affect their libido. Having sex with someone else gives them a temporary sense of liberation. The fear of getting caught causes an adrenalin rush, which translates into an erection. Maybe the two of you are in need of adventure. The security of the relationship is getting boring. You could do something exciting together, something dangerous.
But most women get juicy about security and safety. Dangerous behavior turns them off and dries them up. So most men go out and get some strange to get an adrenalin rush and keep it a secret. It's ironic, but true. They can't help it. It's part of the hardwiring. To expect a man to be faithful is like saying "I won't marry you unless you promise you won't die and leave me because I don't want to be alone." If he promised he wouldn't die, you'd know he was just saying what you wanted to hear, and you wouldn't believe it anyway.
I'm not saying the situation is the same for all men. But it is true for a lot of them. And I'm not saying they can't transcend biology and develop a capacity for deep profound love making that is spiritually inspired, rather than physically charged. But most will need a teacher and lots of experience before they can do that. And sacred sex educators are still viewed with skepticism by the mainstream public in most places.