Seperated

[Dom R]

THe ow is not a [rejection] of you as a person, it is his inability to cope with his depression or whatever isn't working for him,

well, I wouldnt quite go that far. Seems like there is usually SOME issues or two between the two people, even in very clear-cut MLC+affair spouses.
In sideswiped's case, the obvious guess, is that the trigger was the new baby, in one way or another. The usual thing is to focus too much on the baby, and not enough on each other.
babies can trigger MLC and/or affair, for very different reasons.
Spousal focus on baby instead of partner, triggers desires to look elsewhere, sometimes for affairs.
New responsability for baby, can trigger reflective look upon self, and thus sometimes an MLC (I'm a PARENT now?? I'm not ready for this/cant handle it, ....)

TO put it another way: I think that affairs are triggered by lack of something from outside oneself. WHereas MLCs are triggered by lack of something from inside.

(unfortunately, an affair can seem to, temporarily at least, meet both "problems"; by providing external 'validation' for the person having problems)

Dealing with an cheating spouse with an MLC, is double bad news;
Firstly, it doesnt mean that you have no work to do "on you".
The second hit, is that even if you DO clean up all the stuff on "your side"... your spouse may be too self-absorbed to care at this point.

It's not 100% clear that her H is actually in MLC. it may be that he just has re-worked his circle of friends, to those who support him doing something completely imoral and selfish.
Those kind of people, tend to be "bachelor friends and younger girls", too.

There arent too many people over 30, comparatively speaking, that support that sort of thing. Most people over 30, seem to be a lot more in favour of concepts like fidelity, responsability, etc.

In comparison, there seems to be a whole lot of people under/at 30, with no moral standards these days, and just a "whatever makes you happy" attitude.