The Reality
Men are very weak and vulnerable in this regard. When a sexual opportunity presents itself, every cell of their body is screaming at them to do it.
A man once told me that he had to go out and get some strange, or his wife would think he was having an affair. He said that he needed variety in order to "get it up" for his wife. If he didn't spice things up by stepping out on her every once in a while, he wouldn't be able to perform at home, and she'd get suspicious that he was going elsewhere.
Life is very ironic.
Another man told me that he was having trouble getting an erection with his wife, so he went to see a prostitute. The next time he had sex with his wife, it was like they were on their second honeymoon.
Another man told me that he likes the idea of monogamy and he aspires to it, but he just isn't very good at it. He feels fortunate, because his wife is understanding.
I don't know about your husband, but some men are simply hardwired for promiscuity. That is why prostitution is the oldest profession. Married men who need variety, usually rely on professionals, because it is safer and more discrete than having an affair, which can get rather messy.
Were the two of you fighting when he had the affair? Were they just having recreational sex for the fun of it? Or did she do it for love, or for money?
Would you leave him if he did it again?
What if it turns out he is simply not hardwired for monogamy?
Will you love him, accept him and keep him, if he can be faithful and loyal to you in every other way, but he cannot be monogamous?
What if the only options are a faithful celibate husband, or one who makes fabulous love to you, but he needs to see a professional every once in a while to keep the sex working between you?
I hypothesize that men with large testicles may be more genetically hardwired for promiscuous activity. I say this because there are some species of rodents that are monogamous, and others that are promiscuous. Male rodents of the promiscuous species have much larger testicles than male rodents of the monogamous species.
A friend of mine says that men who are creative lovers, don't need variety to spice things up.