In modern, Western societies, I don't believe that being a virgin is held in as high a standard as you seem to portray. What percentage of non-married 18-year-olds are virgins? What percentage of 20-year-olds? 25?

On the flip side, I think that fathers want to protect "underage" daughters from sexual predators (both older and of similar age), until that point when the daughter is able to make a sensible choice for herself. At that point, it becomes a don't ask/don't tell arrangement.