If you think you've got to threaten your daughter's date then you've already failed as a parent.
I want to say this as nicely as possible, but this is a crock. I have three daughters, one living out on her own now, one in college, and one still at home. They were all raised "properly" with open communication about premarital sex, etc, blah blah, blah. The same goes for the first few boys who appeared to take them out. Anyone who thinks that they do not need to be reminded, and in the case of high spirited boys, threatened, does not know of what they speak. Sorry, that is the way it is. Hormones will overrule rational thought almost every time.
The first time my older two daughters actually went out unsupervised, I took the two boys who arrived (brothers, and friends of the family) aside when they arrived. The girls already knew what was coming. It was very short and simple, and there was no waving of guns, knives, or clubs around.
"There is to be no reckless driving with my daughters in the car. If you have an accident, you had better die, because if you don't, I will kill you."
"My daughters are to be treated like ladies regardless of how they behave. If you have a problem with their behavior, come to me and I will take care of it. If you fail to treat them like ladies, I will kill you."
"Do you have any questions?"
While these boys were somewhat reckless, they behaved like perfect gentlemen on that date and on many others to follow. The one boy dated my middle daughter for well over a year before she broke it off. She is away at college, and he actually lives with us now while he attends another college. I found out later his own father threatened to "cut that thing off if it came out where it did not belong". There was no doubt in either of their minds that I would follow through if they did something to harm my daughters.
Anyone who wants to whine about how inappropriate this may have been can pass on the post and keep on going. I'm not interested in how you might think I did a poor job. I didn't.