My wife can't get pregnant, so we have two simple rules. Be safe, and don't fall in love with the bf so much, that she wants to leave me for him.
Other than that, my wife has permission to have sex with whoever she wants, whenever she wants and as much as she wants. But, in reality, she is simply a married woman who dates like a single woman.
I'm an airline pilot now (retired military pilot). It's not uncommon for me too come home from a trip and my wife is in bed with her bf. If they're going at it, I'll strip and join in. If they're asleep, I'll strip and slide into bed with them. No big deal. I enjoy it as much as she does.