Ted is perfectly capable of lying when he needs to. And there's a pretty simple rule for it too: it's not really lying if they don't deserve to know. Lying to Rupert about talking to Rebecca? Easy enough. But lying to Sassy about not seeing anyone... First of all, he trusts her discretion enough that she wouldn't say anything. Second, he feels like he's leading her on or something, even if he isn't really.

When Joan says she wants to be with him, his heart swells. Not just because it's a relief, but... It just feels so good to hear her say that.

"Let's be awkward," he agrees with a laugh. "Although... Let me tell her first, I feel like she deserves to be told." Since he did just lie to her, he doesn't actually want her finding out by watching them.