Hey purple peony. Hope you are all manicured and toned and have been attentive to your OH today (given that you are always totally compatible in every way, of course) because I'm going to send an 18 year old hottie round to see if he wants a bit. And if he does, it is your fault for not being younger, fitter, and generally better frankly 
Now, do you see the flaw in your logic??? (and breathe)
Okay OP, some of the advice on here has been a bit cock-eyed to say the least. Firstly, this is not your fault but completely hers. No marriage is perfect but if there are problems, responsible sensible ADULTS talk about them and sort them out, working on the vows they made and maintaining their integrity.
If there had been problems you'd talked over, been to Relate about and still you were selfishly refusing to change, then maybe she'd have a case (though leaving you first would be preferable) but clearly that isn't happening here.
PP is trotting out a bunch of old cliches that people have traditionally used to maintain the hope that they won't be next to suffer this. Frankly, you could have the best marriage ever that is the envy of everyone - and then someone else comes along who your partner also finds attractive (because let's face it - with all the billions of people in this world there will always be more than one) and so they stray because the other person is simply new.
For instance: they can tell them the stories you've heard a thousand times and get the laughs, they can feel that sensation of exploring something unexplored - and always remember, the first few months of any relationship are usually pretty great. it's only after 6 mo or so that you find Mr or Ms Wonderful is actually a complete dick who chews their toenails or shouts at waiters and the doubts creep in, isn't it?
So - you must move back in and tell HER to bugger off as the price of treating you and her marriage and kids with such a lack of respect. She can come back when she has sorted out what she wants to do next, but she only gets you if she gives him up and agrees to work on the marriage and which of her shortcomings made her do it (could be she feels a bit old and he made her feel desirable, for instance - which is still no justification for being arrogant and selfish enough to break up a relationship and destroy children's lives for the sake of a quick ego boost). Oh - she also has to have an STD check as I bet she's not the only lonely housewife he's been at it with.
I'd personally tell his wife, if only so she can make an informed decision on whether to stay with such a twat. Others here have different views I know.
In the meantime, keep posting. Many people here have been through this and can help.