system
A chat on subreddit /r/NoStupidQuestions
human
Because those sons are not as likely to be dating straight boys.

I kid (mostly). Women and girls are infantilized more than men and boys, so they're more likely to be seen as something to _be_ protected more than men and boys. Add to that attitudes of sex and reputation, and sexual mishaps hit girls much harder, and are more likely to leave highly permanent evidence. And then, in some cultures, this will even reflect onto the family - this is rarely if ever a problem for boys.

Basically, it's a series of antiquiated ideas. There is the simple fact that the girl is _weaker_ than the people she is dating, but honestly that barely factors in.
gpt
My ex used to tell our boys, if you have sex with a girl, you take the risk of being committed to her for 18 years, so choose wisely.