I guess that's where we differ. I'm not religious and find religion stunts common sense. Personally I think that if both people in a relationship communicate and agree on things, they can be much more flexible on what the true meaning of their relationship is.
I'm not religious either and I agree it's to the couple to define what is good for them. In the case I was talking about, the husband know nothing about it, she sleeps with him thinking about fictional characters or anybody else, but him. I find that quite lame (at least be honest and let him do the same). But it's my opinion. I'm not married and don't want to, I allow my BFs to be flirty a bit, to look a bit, but if I sense their mind is elsewhere all the time, I let them go. Open relationship is not my thing.
To come back to cheating. I don't always feel sorry for the person who is cheated on. For instance my brother was dating a girl for several years, maybe 4 years, but neglected her all the time. She was practically running after work to see him and he was barely seeing her too busy with his Playstation. Their time together looked pretty much like her looking at him playing. One week-end they had decided to go to a party with friends but at the last minute he dumped her for the Playstation and told her to go alone with her friends. She was so mad that she drank too much at the party complaining to anybody who wanted to listen and of course there was a guy "nice" enough to listen to all her cr** and console her. Next morning....She came back crying and admitted having passing the night with the guy, she regretted it, she didn't want to, the guy meant nothing to her...and so on. He just kicked her out. Nothing else. Then my mom sided with him to call her a sl**. It was too much for me, seriously I think it was his fault, he neglected her then let another guy took advantage of her, because right, the other guy was not that nice, he was just preying on an easy prey. She was not an easy girl but that night she was vulnerable and yes she cheated on him, but I don't think she deserved being treated that way.
Second case, I know (another) married woman who literally refuse being intimate with her husband, she wants him to take care of the kids, of the house, of her, to listen to ALL her problems (and she has a LOT to complain about) but no kisses, no sex, because she doesn't like that much, or it's too much troubles or whatever, although it was not like that at the beginning of their relationship. He proposed her couple therapy, alternatives (no idea what it is), nope, she even hates when he's like that because no manly enough (?). She doesn't want to divorce because he's a perfect husband after all and I know he won't too because: he loves her and he's a family guy, he wants the family to stay together. So they won't divorce, they won't have sex or anything intimate, but it's still something important to him and while that time, mrs. is flirting with guys on Internet and told me fantasizing a lot about an ex-lover...just not her husband. She's sure he will never cheat on her... Well if he does I might turn a blind eye on it.