Got stuck on the same thing. The quest tooltip is missing a step. After she first comes and shows the plug to you, you have the "Wait for her to approach you" ch 7 quest, but it sticks. I went through a TON of other content until it was the last thing I had, then started trying to see what would trigger it. It looks like you have to be the one that makes the move. I'm not sure if it works with all of the "Initiate sex" actions, like when doing dishes, or if its at a certain time of day, but I approached her when cleaning the bathroom I think in the evening, told her I wanted to go to her room, and did NOT choose the new blouse option. It then triggered the plug sex scene which cleared the quest.
I had done plenty of repeat sex scenes with her previously, but had chosen the "Wear the blouse" option which triggers a different set of scenes, so I think by choosing that one, it failed to trigger the plug scene after the initial one, where she shows that she is curious.