In most cases, it is simply because they aren't getting enough. The urge to have more sex is generated in the same part of the brain as the urge to have another slice of chocolate cake. They also wanted a person who was not a threat to their wives or families.
Why get married if your imtention isn't toward monogamy?
Men are still biologically driven to provide for their females and ensure their children are safe. Marriage continues to provide the social, legal and security benefits. The introduction of sexual exclusivity to marriage was originally intended to ensure inheritances stay within families, if you use a non-biblical rationale, but given that the modern Will and Testament, monogamy is no longer relevant for that purpose.
Can men really be monogamous?
I would say 100 percent of men cannot be mentally monogamous, in that they are biologically programmed to assess the sexual attractiveness of every single woman they see. A normal man could also achieve monogamy with a highly-sex driven woman, as could a man with a low libido and a normal woman. However, a normal man and a normal woman who have decided on lifetime sexual exclusivity [could] face chances of divorce and a very high chance of cheating.
NO...However, you can use his nature to work for you, rather than against you. That means negotiating some infidelity if you are not “in the mood.” It does not have to be other lovers: It could be pornography, or a lap dance [at a strip club]. The important thing to remember is that "sex" is your man's strongest, most basic instinct, so working with it puts you in a position of power in the relationship. It is better to walk the dog on a leash, than to let it escape through an unseen hole in the back fence.
Also, never cross your legs for an extended period, that's opening the door to infidelity. Don't let yourself go, always keep yourself up. There is nothing less valued than a woman that no one else wants. Always remind him how valuable you are, how popular you are and drop "subtle" hints that other men still find you desirable. This will take advantage of his biological need to compete.