I would argue that both sexes are essentially hypergamous during a relationship, with or without a marriage contract. Statistics would also support this. There are many men here that will sleep with a taken woman, regardless if she is married or not. So, for a man to sit back and bed all these women with boyfriends, husbands, SO, or whatever, then expect it not to happen at all to him is downright ludicrous...
When a partner (man or woman) meets someone new with interest, they begin measuring up to their current relationship (hypergamy). If they do not measure up, or meet the qualities of their current partner, they will retreat or LJBF. However, if the new interest has "potential" to surpass their current relationship, the dance of relationship death begins. The excitement, the push and pull, the invigorating sex, etc etc... If the new interest reciprocates, then at some point, the partner has to make a decision to stay or leave. The point when someone decides to "upgrade" or "branch swing" to a new mate is the same point when they decided their current mate no longer provides as good of value.
If the partner "upgrades" you and is completely unexpected, you failed at filtering partners properly AND being cognizant of your shortcomings in the relationship.
What
@guru1000 says is don't be that guy that has
a) let his/her value diminish during the LTR,
*people get lazy in relationships
b) never had much value in the first place, maybe hooked the woman for a number of reasons...
*people put on a facade, one that will only last for so long
This is why you need to be the man she fell in love with (not the simpering fool with the pretty girl) AND give relationships time to develop, so they can truly reveal themselves. You should be the one making the decision when it's over!
Hypergamy is alive and real, but it is part of natural selection, part of our biology, human beings just can't help it. So, if you are always looking for an upgrade, why shouldn't she?
If you are the man she can't upgrade, or even given the opportunity to think it, you will ALWAYS have the upperhand