'Call me mundane, but even if I stretch my imagination to the limits, I cannot see how anyone who really cares for and loves another person is comfortable with them having sex with someone else.'
Just because YOU cannot imagine it, it must be wrong? If I thought the same way, I'd have a serious problem with lesbians, because I cannot imagine enjoying sex with a woman. And because I can't imagine it, it must be wrong/weird. 
But I don't think that way, because what consenting adults do is their business, and I firmly believe with all things out of the 'norm' of vanilla, monogamous relationships that those people haven't made a CHOICE about what they enjoy sexually--they have just admitted what/who they like and what feels right to them! If everyone did so, the world would probably be a better place.
cabbageroses, those married women should be addressing issues of monogamy with their husbands, not getting pissed off with whomever their husbands shag. Totally agree, if someone chats you up, you are not responsible for interrogating them. It is completely reasonable to assume that by the act of chatting you up, the person is saying 'I'm free to do what I like'. Do you honestly think every time a guy chats someone up, that woman should say 'are you sure you're single? you're really not married? Don't have a girlfriend lurking somewhere?'