If the sex had stopped in our relationship and we still were very in love and wanted to stay together but felt sexually unsatisfied, then I would talk to my OH about "cheating" though I don't think it can really be called that when you both agree that it's a good idea. But if the idea didn't go down well, then further actions down that line would feel like a betrayal. And if I was the one frustrated and my OH said "no" to me being with other people, then it'd be down to me to weigh up how important sex was compared to my relationship with my OH.
I don't think I could ever be with someone other than my OH without him knowing and understanding why, i just wouldn't be able to go behind his back, look him in the eye and lie about it. Not without experiencing some extreme depression because of it.

But that's just my