It's not a case of putting my fingers in my ears, because it simply doesn't hold true in reality.
Sure, there is a type of girl that is always out to get the bigger, better deal, but you can't make the assertion that ALL women, regardless of age, culture, religious beliefs, etc. operate solely with this mentality.
If that was true, why would any women stay married to a man after he became physically disabled? Why would a woman stay with a guy that had a terminal illness? Why would any attractive woman stay with a man that's out of work, when she knows that she could attract a wealthier guy? How can ANY marriage survive after the initial attraction wears off?
It all comes down to the personal values and character of the woman, as well as her respect for you. A woman with strong personal morals will not cheat on her husband. She WILL still feel attracted to other guys, just as all of us will often feel more attracted to a girl that is not our girlfriend. But a moral woman will not cross the line. It's equally as important that a woman truly RESPECTS her man.
ATTRACTION gets the girl, but RESPECT keeps her. I did a thread about this here:
http://www.sosuave.net/forum/showthread.php?t=149319
The problem is that guys too often view marriage as "ownership," with no thought given to upholding their end of the agreement. These guys let themselves go and tolerate disrespectful behavior from their wives, because in their mind, they've already "won" her. This was sufficient in the past, but as the marriage contract becomes weaker and divorce becomes more readily accepted as a solution, this doesn't fly any more. I think you stated this stuff yourself, so we're in agreement on a lot of this--but I think we differ in opinions as to what keeps a girl in a relationship.
The proverbial bigger, better deal will generate ATTRACTION in a woman, but attraction usually doesn't translate into action. I think we can safely assume that a woman that's been married to a guy for 10 years won't feel the same level of attraction for him as when they first begun dating. We can also assume that at some point in those 10 years, assuming that she's attractive, she's been approached by a more attractive, wealthier man. We can even assume that she's had "crushes" on other guys. That doesn't mean that she's open to leaving her husband.
The raw truth is that strong personal morals and standards, coupled with respect for her mate, are going to trump any supposed "hard wired" urges that a married woman has. That's why you have to choose wisely if you want to get married.