What you have here is a classic Schedules of Mating dillema.
All you're seeing is methods women have used for centuries to ensure that the best male's genes are selected and secured with the best male provisioning she's capable of attracting. Rarely do the two exist in the same male (particularly these days) so in the interest of achieving her biological imperative, and prompted by a biologicaly seated need for security, the feminine develops social conventions and methodologies (which change as her environment and personal conditions do) to effect this. Men are not only up against a female genetic imperative, but also a centuries long feminine social convention established from a time long before human beings could accurately determine genetic origins.
I've aleady detailed in many prior posts that mate selection is a psycho-biological function that our millenias of evolution has hardwired into both sexes. So internalized and socialized is this process into our collective psyches that we rarely recognize that we're subject to these motivators even when we continually repeat the same behaviors manifested by them (such as having the second kid with the Bad Boy). So saying that we're not subject to conditions we're not, or are only vaguely, aware of is a bit naive.
It's simple deductive logic to follow that for a species to survive it must provide it's offspring with the best possible conditions to ensure it's survival - either that or to reproduce in such quantity that it ensures survival. The obvious application of this for women is sharing parental investment with the best possible mate her own genetics allow her to attract and who can provide long term security for her and any potential offspring. Thus women are the filters of their own reproduction, where as men's reproductive methodology is to scatter as much of his genetic material as humanly possible to the widest available quantity of fertile females. He of course has his own criteria for mating selection and determining the best genetic pairing for his reproduction (i.e. she's gotta be hot), but this criteria is certainly less discriminating than that for women (i.e. no one's ugly after 2am). This is evidenced in our own hormonal biology; men posess 17 times the amount of testosterone women do and women produce substantially more estrogen and oxytocin than men.
That stated, both of these methodologies conflict in practice. For a woman to best ensure the survival of her young, a man must necessarily abandon his method of reproduction. This then sets an imperative for him to pair with a woman who will satisfy his methodology. A male must sacrifice his reproduction schedule to satisfy that of the woman he pairs with. Thus, with so much genetic potential at stake on his part of the risk, he want's not only to ensure that she is the best possible candidate for breeding with, but also to know that his progeny will benefit from both parents involvement.
One interesting outcome of this psycho-biological dynamic is men's ability to spot their own children in a crowd of other children more quickly and with greater accuity than even their mothers. Studies have shown that men have the ability to more quickly and accurately identify their own children in a room full of kids dressed in the same uniforms than the mothers of the child. Again, this stresses the subconscious importance of this genetic trade off.
Social Convention
To counter this subconscious dynamic to their own genetic advantage women initiate social conventions and psychological schemas to better facilitate their own breeding methodologies. This is why women always have the "prerogative to change her mind" and the most fickle of behaviors become socially excusable, while men's behavior is constrained to a higher standard to "do the right thing" which is invarably to the advantage of a woman. This is why guys who are 'Players', and fathers who abandon mothers and children to pursue their innate reproduction method are villains, and fathers who selflessly sacrifice themselves financially, emotionally and life decision-wise are considered heroes for complying with women's genetic imperatives.
This is also the root motivation for female-specific social dynamics such as LJBF rejections, women's propensity for victimhood (as they've learned that this engenders 'savior' mental schemas for men's breeding schedules - Capn' Save a Ho) and even marriage itself.
Good Dads vs Good Genes
The two greatest difficulties for women to overcome in their own methodology is that they are only at a sexually viable peak for a short window of time (generally their 20s) and the fact that the qualities that make a good long term partner (the Good Dad) and the qualities that make for good breeding stock (Good Genes) rarely manifest themselves in the same male. Provisioning and security potential are fantastic motivators for pairing with a Good Dad, but the same characteristics that make him such are generally a disadvantage when compared with the man who better exemplifies genetic, physical attraction and the risk taking qualities that would imbue her child with a better capacity to adapt to it's environment (i.e stronger, faster, more attractive than others to ensure the passing of her own genetic material to future generations). This is the Jerk vs. Nice Guy paradox writ large on an evolutionary scale.
Men and women innately (though unconsciously) understand this dynamic, so in order for a woman to have the best that the Good Dad has to offer while taking advantage of the best that the Good Genes man has, she must invent and constantly modify social conventions to keep the advantage in her biological favor.
Reproductive Schedules
This paradox then necessitates that women (and by defalut men) must subscribe to short term and long term schdules of mating. Short term schedules facilitate breeding with the Good Genes male, while long term breeding is reserved the Good Dad male. This convention and the psycho-social schemas that accompany it are precisely why women will marry the Nice Guy, stable, loyal, (preferably) doctor and still ƒuck the pool boy or the cute surfer she met on spring break. In our genetic past a male with good genes implied an ability to be a good provider, but modern convention has thwarted this so new social and mental schemas had to be developed for women.
Cheating
For this dynamic and the practicality of enjoying the best of both genetic worlds, women find it necessary to 'cheat'. This cheating can be done proactively or reactively.
In the reactive model, a woman who has already paired with her long term partner choice, engages in an extramarital or pairing, sexual intercourse with a short term partner (i.e. the cheating wife or girlfriend). That's not to say this short term opportunity cannot develop into a 2nd, long term mate, but the action itself is a method for securing better genetic stock than the committed male provider is capable of supplying.
Proactive cheating is the single Mommy dillema. This form of 'cheating' relies on the woman breeding with a Good Genes male, bearing his children and then abandoning him, or having him abandon her, (again through invented social conventions) in order to find a Good Dad male to provide for her and the children of her Good Genes partner to ensure their security.
I want to stress again that (most) women do not have some consciously recognized, master plan to enact this cycle and deliberately trap men into it. Rather the motivations for this behavior and the accompanying rationales invented to justify it are an unconscious process. I fervently believe that for the most part, women are unaware of this dynamic, but are nonetheless subject to it's influence. For a female of any species to facilitate a methodology for breeding with the best genetic partner she's able to attract AND to ensure her own and her offspring's survival with the best provisioning partner; this is an evolutionary jackpot.