Do you honestly think it is in a child's best interest to be in a family where the parents stay unhappily married? I know how badly this can end. One of my closest friends was completely sideswiped when his parents split after he moved out of the house. Apparently they made an agreement to stay together for him, but once he moved out they were allowed to go their separate ways. He was devastated and felt completely betrayed and to this day (4 years later) he will not even speak to his father.
Granted this is an extreme, but come on now, staying together is not always what's best for the children involved.
The point of getting married is to be with someone who you deeply love and want to spend your life with. That doesn't mean that person can't change into someone else. In my situation, my husband stopped coming home, developed a close relationship with another woman, and didn't understand why I would be upset by this, or maybe was just too oblivious to care. I spent months trying to fix things, but when you're the only one trying there isn't much else you can do. So yes, when my husband told me he "fell in love" with another woman, I told him to pack his bags and get out of the house. Even after that, I still took him back when he said he wanted to work on things and promised me he would make it up to me. Which he apparently thought meant continuing his "friendship" with the other woman, continuing to ignore and fight with me about "what else I would take away from him" (besides his g/f), and eventually, yes, I kicked him out of the house again and we got a divorce.
I didn't get married expecting that the person I was in love with was going to turn out to be neglectful and spiteful. Nor did I realize after the split he was going to tell me he never wanted to get married (why propose then?), never wanted kids (deal breaker for me), and thought that I was a freak for any number of 'odd' things I liked and/or did. Was I supposed to stay married to him? Granted, no, I didn't have kids (of which I am insanely grateful for), but even if I'd had kids, I don't think it would have been fair to me, or to them for me to stay.
I realize the situation in this thread is very different from my own, but the point is the same that you can't just blanket-apply your feelings to say that all married people should stay married, or all people with kids should stay married. Every situation is unique, every person is unique, we are allowed to want what is best for us; we are allowed to want to be happy.