BP Totty: I’m agree with you that we are not tuned into the importance of preparing our sons for marriage with regard to developing their maturity level/making them into a mentch, in the same way we are tuned in towards our daughters.
However, with regard to the reason why nobody is against boys-EY, I would go with choice A: People want to believe in the illusion that everyone/most boys in EY are shteiging away, since to think/believe otherwise would be considered as thinking too cynical.
Additionaly, girls remaining home that year can actually help prepare them for marriage, however boys remaining home will not contribute to their maturity, as they will regardless remain in yeshiva all day learning w/o dealing at all with their future responsibilities in upcoming marriage.