Heavy, take a little bit of musclemom dutch uncle advice here from a woman whose been there, done that, got the t-shirt and married the man I cheated with and am happy with after 7 years of marriage.
JUST GET THE FUCK OUT. Seriously. The single most ENORMOUS regret I have in my life is staying in my first marriage when I knew that a) I was no longer in love with him and b) there was no way for me to recover those feelings.
My current husband and I missed out on time we could have been spending together staying in marriages that we didn't have any heart for, mainly because of kids and money and some extenuating circumstances. The result was we ended up creating even more anger and heartbreak and bitterness on the parts of our ex-spouses.
I don't feel badly about leaving my ex-husband, I feel badly that the marriage didn't make it and I feel genuine shame that when I realized it was over, I didn't leave him righteously.
That being said, the comments about women having virtually a 6th sense about cheating, uh, yeah, that's true. You might get away with a once or twice a year fling, but a sustained affair will be discovered within a year, no matter HOW slick you are.
Now, can it be done if you have a lot of cash at your disposal that won't be missed, and a reason to be away from home for extended periods of time on a regular basis? Yeah, that's how a lot of polygamists get away with having two or more families for decades (the major drawbacks to affairs are money and where to get together, unless you're seeing a single person). But having a regular thing with someone local, particularly if this individual is also married, is doomed to discovery sooner or later.
And seeing a single woman is doomed (and just really shitty), because sooner or later she's gonna be sick of playing second fiddle and putting her life on hold in the vain hopes that you'll leave your wife for her.