I encourage my missus to do so - but she doesn't often take me up on the offer! Turns me on terribly to think of her fucking behind my back. I never pressure her though - entirely her choice.
If your swingers its not cheating, but if you mean in general if you have to cheat there are problems, uf the problem can be sorted fix it, if not it's time to move on. Never cheated it's not necessary.