In the novel it was mentioned, that the previous family hired some old lady who teached the Mc how to conceal his gender while having sex. This is how he could dance too. This made a lot of more sense since it was strange that the family seemed not to give a shit that they tried to betray a prince without much concerns of the outcome.
The reason he could offer a position to a man that is offered to women is because literally everyone thinks Yu-er IS a WOMAN. No one is really questioning if he is a man, nor does anyone need to. So as far as they can tell the prince is offering a woman a woman's position. I don't really understand the first part myself too Much either.
Okey I like it but I am confused. I understand when he had sex with the yin (sorry if I got wrong) is obvious that he knew he was a man but didn’t he though that he was trick or something like he was marrying the daughter n now he got a man, sure he is beautiful but still. Second, is the father giving the prince what he wanted out of the marriage. Third, that stupid doctor let slip that yin is a man, n how in the world would u offer a position that in those time belongs to a woman. Like is a good story but I need more explanation...