BTW, open advice for married guys going through this.

I just went through it last year. Twenty years of marriage, and suddenly she's digging up ex-boyfriends and sleeping with them. We powered through it without a counselor (though not getting counseling was a huge mistake - I tried; she refused; what are you going to do?)

Here is what I learned, and want to share with you guys:
All of this advice is about what is; not what's "right" or "fair"
There will be much stereotyping involved. Take it with a grain of salt. But I've seen this pattern repeat enough that it's worth laying out the stereotypes.
In a one-income family, where the man works and the woman stays home, parents, takes care of the house, etc - the guy is going to feel like everything is okay. He's working his ass off to provide for the family; he's doing what he can for the Mrs. He's spending the few hours he has at home unwinding (reddit, football, xbox, etc). She sees this as emotional neglect.
We fall into habits, and get complacent with our relationships. To us, the marriage is like an old easy chair - comfortable, and there when we need it. To her, if it's not fed and watered regularly, it's in a state of neglect and abandonment.
If you have kids, when they become independent (bike riders, drivers licenses, college, etc) then the primary parent is left in a vacuum with way too much time to think about what they hate about their lives. You may easily become the target of this malice.
I wrote a note a while ago about the "men are from Mars" factor of complaining. When men complain, we're looking for help. When women complain, they're looking for empathy. When a woman complains to a man, she's looking for a shoulder; but we think she's looking for help. So we offer to help. This can be perceived as "attempting to run their lives" - another black mark.
If she complains about the way things are, and you try to offer your perspective, if you're not careful, this can be viewed as "twisting their words around" - another black mark.
If you're unbalanced in the libido department (very likely if she's being a mom), then your "nagging" about sex will be perceived as "he only wants me for sex" - another black mark.
All of this stuff adds up and makes her an easy mark for ... the playa. Guys who cruise cougars and know exactly how to get them into bed. Watch "True Lies" for a caricature of this jackass. He woos them, dates them, listens to their problems. He won't slam you directly, but he'll draw her out about her issues with the marriage. Meanwhile, he's subtly suggesting what an awesome guy he is. Voila - they're meeting at Motel 6.

How do you combat this? If you've been married a few years and things seem normal, here's your checklist:
Do the dishes. There should never be dirty dishes in the sink.
Make the house a home. Tidy up at night; close the blinds; turn on some lights. In the morning, lights off, blinds open. Buy some plants and keep them watered.
Don't walk past anything that needs doing. Straighten throw pillows. Make the bed. Clean up your dirty laundry.
Fix those things you always said you'd fix.
Every so often get up early and make sure she wakes up to a clean house. If you're a night owl, clean up after she goes to bed.
Spend time with her. Take her out to lunch or dinner at least once a week.
Buy her small presents for the hell of it.
Does she have her heart set on some major home renovation? Give up the new computer or car and budget for that.

This isn't a band-aid; it's a plan for the rest of your life. Do these things to make her happy, even though she won't really notice or acknowledge it. What she will do is relax more at home and be generally more at peace.

If you're already in the danger zone, do all these things and also start talking to her about your marriage. Talk long term plans. Apologize for being absentee husband. Take the blame a lot. Listen to what she says. When you try to explain things from your perspective, make it clear you're not trying to assign blame or establish right or wrong - just that it's your perspective.

Finally, for the women who might find a hint of familiarity with this - talk to your husband. If you're not happy, talk more. When the asshole at work or on facebook or in your sewing class starts showing you more attention, notice if he's paying more attention to you than anyone else. If so, assume that he's just looking to score with you and nothing more. If you are truly that unhappy with your marriage, you must go through a two-step process:

1) Would you be happier alone than with your husband? If so, leave him.

2) Once you're single, then you can start dating again.

You should never compare your husband to some guy who's pursuing you - it's an apples to oranges comparison. And by the way - if the guy pursuing you is married, he's cheating on his wife, you idiot.

One final thought - once upon a time I was told that any person could write down ten qualities they look for in a mate. If you are wise, you will marry someone with six or more of those qualities. When you cheat, you are pursuing one of the "missing" qualities. So when you find yourself tempted, think about the whole list, how much of it is satisfied at home, and what exactly you're pursuing.

And, of course, my axe.