NYS, keep in mind that not every straying spouse has the affair because the marriage was bad.
Yes that's true, yet I didn't say that this was the case with every straying spouse. I wrote that it was "very common". When I hear an innocent partner state, "I thought everything was fine", that sends a flag to me that it may very well be this common situation. Typically, the soon-to-be straying spouse has long given up on the relationship, unbeknownst to their partner. Their partner, not hearing complaints, nagging, whining, demands, etc., mistakenly thinks their spouse is content. When the bomb drops, the innocent partner is totally bewildered, not having seen it coming at all.
some do it for the prestige of having a younger or more attractive companion, etc. --------------------------------------------------------------------------------
Ok, in this case, which seems like my W's case, how do we combat this? I can't become younger or more attractive. I can get in better shape, but once that's done, and it is, then what? If she wants the younger, bigger, better looking guy, then am I just stuck hoping he turns out to be a total a-hole?
If the cause of the infidelity was because the innocent partner got out of shape and became physically unattractive (which then leads to any number of other disappointments in the relationship), then getting back into shape can bring the attractiveness back. Our society labels such a thing as being shallow or superficial, but the truth is that physical attractiveness in a partner is a human need, and to battle needs on a philosophical level denies their importance and reality. We wouldn't argue against other physical or topical traits, such as poor hygiene, being unkempt or boorish manners. All these things can also have a negative impact on the sexual life of the couple as well, as it can diminish desire.
There will always be others who are more attractive in any manner of ways, we are not in competition with them. What you can do with it is determine what it is that influences the WAS, or rather, how the WAS may influence you, rather than you being polarized and digging one's toes into the sand, for in a relationship, the partners help each other to develop those aspects that are underdeveloped in each other.
You can lose weight if you're obese. You can dress nicer if you're a slob. You can bathe more often if you stink of body odor. You can welcome some new agreeable sexual behaviors into the bedroom if sex had become routine or not satisfying. You can expand your horizons in other interests if you became a one-trick pony or a couch potato. You can cultivate better manners, better habits. All these things must be about development of self, not about competing with any OP.