He knew it was a joke and that he was just being teased XD
İ think he realized she is suspecting him to be not eunuch (because real eunuch shouldn't be worried about infertility since they already infertile) so he should have drink to remove suspicions
OHHHHHH
My reactions are similar when i read insights of this manga. Very well thought. İ believe there are even more details in novel
mmmhhhh why jinshi drink the tea at the end...? isnt mao mao said it can cause infertile to male?