Did he just want to look? Sansa has no idea what he's about but when he asks for her to spread her knees wider, she does so, and looks down at him quizzically. She braces her hands against the table, too, so she isn't in any danger of falling off of it. She doesn't think she is but she has no idea what is about to happen.

"No one has ever looked there. Not even a maid. Not even myself. You'd be the first person to...see anything."

Sansa isn't a complete idiot and she does know that her husband is meant to push himself inside her there so Daemon would have to look eventually but she thought he'd save it for the wedding night.