I must admit, forgiving somebody because of their gender is pretty crass. Yeah, I suppose it's reasonable to "forgive" a guy for not knowing all about menses, just like it's reasonable to "forgive" a woman for not knowing about jock straps, but to me it just sounds arrogant.
"Be forgiven for not knowing" is an English idiom that probably doesn't translate well to other languages. In this case, it comes from a certain amount of poking fun at somebody for not knowing something by "forgiving them" as if they had committed some sin or faux pax rather than simply been ignorant.
Maybe it was funny once, a century ago. Now it's just arrogance.