Biology and a lack of self-discipline
What does biology have to do with him being a lowly degenerate? If he wants to spread his sperm around so be it, he just does not need to be a fucker and get into a relationship knowing beforehand that he will be cheating on his partner.
In short, stick to one night stands if you're big on teh biology.
Men are biologically geared to spread their DNA to as many potential mates as possible. It's a simple function of evolution and the same holds for 99% of the mammal species on the planet. Monogamy is an extremely rare trait in nature. In fact, Polygamy is more common in human societies than Monogamy.
I agree with the consensus that if you agree to be monogamous with your mate and you're incapable of doing so, you shouldn't be in a committed relationship.
I do agree that humans are most likely not a monogamous species, which is kind of irrelevant to be honest with you since humans' heightened and exceptional state of consciousness or cognizance and sapience elevate the individual above mere biological or imprinted functions. In any case just like you perfectly put it, you shouldn't enter into an agreement or social contract with someone knowing that you would not be honoring it.
Our heightened self-awareness is a biological function