I wouldn't really care that much.  It might not be my first choice but whatever.  In reality, there isn't a hell of a lot you can do to stop them if that's what they want.  Telling them not to date x,y,or z person will only make them want to do it even more.
It's best to make peace with that and hope they don't come home with a total piece of shit.  I'm glad I don' have a daughter.  Total nightmare.