While generalizations, a LOT can be explained by the study of roles throughout time, and in the animal world... Yes, we are animals!!
VERY few animals MATE for life. I'd say that WE are not a species that necessarily does just based on the current divorce rate.
What R the primal forces that motivate both sexes and what are the roles if we break it down to a more primitive level? Well survival of the species suggests the urge to breed is instinctive. While it may not be as simple as that, the reality is that men can produce children throughout their entire lifetime, and could produce hundreds of children by many different mates. In fact in some past cultures men have done exactly this.
I would also throw out that a man is VERY willing to *breed* with a woman he may not necessarily like if he finds her attractive (and maybe not even that) where as for *many* females I believe that they rarely have *physical relations* with someone they are not interested in just for the sake of *the physical*.
I'd suggest that many men after a breakup will seek acceptance in another woman's arms knowing that the relationship may not last, or even work. Woman OTOH, often do not want much to do with men close to a breakup for a variety of reasons.
I belive that men tend to be more selfish than woman (another HUGE generalization). Orlando Rouge said she did not think it was fair to a potential new partner to start a relationship close to breakup. I do not think that this would enter into the equasion for most males.
I'm married with children, and have been 17 years BTW in case you are wondering.....
Ron