OH, and for the love of God DO NOT STAY TOGETHER FOR THE CHILDREN. Nothing screws kids up more than being raised in a loveless marriage. I was the product of a divorce, my wife was the project of a marriage that stayed together for the kids. Took me years to teach her that marriage didn't have to mean the end of happiness (our first few years together were an adventure) every few months another what if from her screwed up childhood would occur to her, she wouldn't mention it until it had driven her to nuttiness then I had to difuse the mess. Other people I know raised in similar circumstances have gone through the same thing, most are afraid of any form of emotional commitment because their experience says commitment equals misery.
Another factor is that if you stay together for the kids you'll begin to resent them unconsiously, and they'll pick up on this. And that hoses the kid up to. They don't understand what's going on, all they know is their parents are miserable and seem to blaim them for it, so they accept responsibility for it and carry the guilt for years.
Sometimes a marriage can be SEXLESS, but the partners still have a high regard for one another and raise the children well. Should the man (who still needs and wants sex, but just ISN'T EVER going to get it again from the MRS.) just toss the marriage on that one ground, or, with the consent of his wife, DISCREETLY get a girl on the side????