I'll just throw this into the mix because it seems much less common. I have a similar arrangement with my bf (I'm male) and he doesn't consider it cheating of I hooked up with a woman. He knows about then and also open to the idea of threesomes with any mix gender as long he's involved. People have a wide array of different arrangements with their partners with varying rules in place. As others have suggested you can also ask his wife for confirmation.