Personal views aside, as a writer I have to give two seperate views on this having written a story where infidelity was key to the plot (and in my opinion, the reasons for it were valid if undesirable) and where the other partner 'put up' with all the (dire) consequences thereof because he understood why it happened, no matter how much he hated it and ultimately how much it cost them both.
On the other side, in the main one I am doing at the moment, fidelity (between the [male] couple) is absolutely key. Without it, the point would be missed entirely.

I'd say it depends on the story. I mean, Rafe in Standish was annoying because he couldn't keep his trousers on, but it was part of the story. You wanted to reach into the book and slap him, but at the same time, you wanted him to git nekkid with teh Italian...*coughs* : )
Personality of the characters is significant. If everyone was always faithful it would soon seem like you were reading the same story again..and again...and again.