Actually, Yao Ying wanted them to take a bath together despite knowing that he would be in pain was because of an important reason --- not because of his dirty thought. Before he told Yao Ling anything about the crescent moon birthmark, he just wanted to make sure that what he had seen wasn't fickle of his imagination...
Maybe he was feeling too pleasurable at that moment until he was seeing things?
Yao Ying removed Yao Ling's hair to the front slowly, so he could take a look at the back of her neck, trying to find a clue. He tried to do it without making Yao Ling feel suspicious. True enough, there was nothing there. He wanted to sigh in regret, however, when he squinted his eyes, he saw something there...
Oh well... It seemed like he didn't see it wrongly...