To be fair, Ted can't imagine being naked with Joan without intimacy being involved. And sex, for him, is inherently a form of intimacy. So it's sort of the same thing.

He's never personally understood how you can enjoy having sex without having at least some of that intimacy and trust - if not romantic, then at very least on a platonic level. He didn't exactly know Sassy all that well, but they were still very much enjoying each other's company, and not just trying to satisfy their own needs. He respects that people might feel differently, but he can't identify with it.

"Does nudity bother you in general?" he asks curiously. He hadn't gotten the impression that she's particularly shy about nudity and sex, but she can imagine she just doesn't consider it any of his business.