I was chatting to a mate of mine several years ago. He was a really nice guy, but also one of those thrusting, City types who made money in the murk of "financial services". He lived in the better part of town, extraordinarily pretty wife, collection of matching his and hers convertibles, etc, etc.
One day while we were sitting by the pool drinking Chablis, his wife told me about the first time she'd introduced him to her mother. Apparently, after dinner he was so lubricated that when he kissed her mother goodbye, he slipped her some tongue.
Fortunately, she says, her mother was so broad-minded that she never said anything. Having subsequently seen her mother, I can see how you'd make that kind of mistake, but I'm not sure I'd do it in front of the woman I was dating.