Tell her you've already had sex

You square your shoulders and try to sound as confident as you can.

"Rebeccah and I have already been...intimate. Is there really a point in waiting to get married now?" you ask.

A change comes over Rebeccah's mother's face, like a cloud darkening the sun. You almost shiver.

"Young man," says Rebeccah's mother, her eyes glinting dangerously, "A vacation dalliance, or a sexual fling is a far cry from what you have signed up for. I would expect my daughter to be 'out sowing her oats', if you will. I am a modern woman and I know from my own experience that women have sexual needs that need to be fulfilled. Regularly."

She pauses, glaring at me as if daring me to interrupt her.

"You are no longer a dalliance, fling, stress reliever, or any other thing like that. You are a fiance, and that is something completely different. I expect you to take this seriously. There will be no hanky panky until the wedding and that is final. If you feel otherwise, the door is right there. And don't think about being sneaky either. Rebeccah learned long ago that keeping secrets from me is very unwise, and I suggest you take heed of that."

"Am I understood, young man?" she asks, perfect eyebrow arched, green eyes flashing menacingly.