Well, according to Andy Bobrow:
Maggie, who wrote the ep, insisted that Jeff’s line “If we were married…” was not flirtatious… The rest of us said you’re insane if you think that. It’s incredibly flirtatious. But Maggie insisted it was not meant to be. Then when they filmed the scene, Maggie admitted it was. Or, I mean, she admitted she saw it differently once it was happening in front of her.
So basically we all thought that scene was the beginning of a build-up but turns out, it wasn’t.