I would cheat, but I would be discreet about it. Discreet, in this case, does not mean secretive. I would not parade my escapades in his face or do anything that would be obviously mean-spirited and hurtful, but I am not going to go without sex. Period. (I would not expect him to go without sex if the situation were reversed.)
I do not know many men (gay or straight) that would. I believe the men who say they would never cheat in that scenario are lying—to their spouse, to themselves, or to both.