Look at her in that frame after the reveal. She's not startled at his manhood. She's startled at her body's response. She splooshed the moment he turned to her full-frontal, and simply had no idea what it was she was feeling.... Such an innocent.
Ahh, this explains a lot. This is the trauma Tsuki-san suffered to make her think Shinichi was horny incarnate, banging her daughter, and was looking for a chance to eat her next...