Know this has been done before, but not for a while:
My m8 Dave confided to me that a relationship seems to be developing between him and a lady he has met recently. He is married and has a child. Apparently his marriage has evolved into just a way of life and he and his wife haven't been intimate for over 4 years (they had to have fertility treatment to have their child which can't have helped). Oddly enough, he said that this wasn't bothering him too much and he had more or less accepted that this is the way life was going to be - he had given up initiating things because he became tired of being rebuffed. But then this lady started to pay him rather a lot of attention and he had forgotten what it was like to have someone interested in him and (apparently) attracted to him.
He doesn't intend to let it get physical (and said that he would end his marriage before that happened), but was wondering whether what he is already doing (going for a drink and meeting her regularly - they walk their dogs in the same place) is a bit far. And he thinks he won't end his marriage as he doesn't want to "do that" to his child, although I question how healthy it is to grow up in a loveless household myself.
4 years. I find that really sad and have mixed feelings about whether fooling around with someone else can really be condemned in the circumstances.