When the lack of sexual appetite in a monogamous relationship is natural and more common than you think, can the couple survive this? The issue is what to do: it remains for our own satisfaction or we jump the fence of fidelity. How each one approaches this dilemma is perhaps the key and differential point: identify the limits, do not cross them, talk with the couple about what happens to us, respect the agreements we made...
or maybe not. It's a very personal response. Each couple is a world and each one solves (or not) their problems within that universe they built between the two.