Children are the property of their fathers. Preserving the value of that property through preserving the virginal status of females is the responsibility of both sons and daughters. For a son to have sex with the daughter of another man, both the daughter and that boy have committed a property crime against the girls father. If she gets pregnant, he will never be able to get as big of a dowry when he sells her into marriage.

At least, that's where the idea of tying sexuality to economic considerations like 'responsibility' and phrases like 'saving yourself' come from.