Well as we all know eunuchs have been castrated, which mean both their testicles have been removed, So they tend to lose the desire for sexual activity but does not eliminate it, so some will still have urges. (But I think eventually they will lose all sexual desires).
So my take on this is that, I think at first Yuern thought KT no longer had sexual urges so that is why she didn’t make any sexual advances towards him at the start, but only wished to be loved by him. But after she saw KT kissed MM that night and after continuing to witness the way KT looked at MM when MM was around him she then realised KT still have urges like any other man (the only difference is he supposed to have been castrated.) That is why the night she tried to molest him, she said to him she knows he still has urges, of the way he looked at MM and knows that he wants MM, so basically she was trying to tell him to think of MM while they Lo Saam Pao.
So when she touched little KT she realised little KT was standing nice and tall and that little KT round twins was still intact so she freaked out because now she knows he isn’t the person who he claimed to be and she is in danger as she has now uncovered his secret.