Okay, this is definitely one of my favorite scenes.
First of all, this scene is one of the first instances where we see Sherlock changing from a great man to a good one. He initially calls Jim gay and shrugs it off as no big deal. Then he realizes Molly hasn’t caught on yet and he hears how hurt and confused she is. You can see the look he gets, he realized he’s said something not good. So he does something very unlike Sherlock. He takes it back. He doesn’t try to prove his cleverness, he just stops. He doesn’t ruin it for Molly. (Not yet, at least)
Also, this scene is very important in the lead up to the Reichenbach Fall. Moriarty says Molly has told him all about Sherlock. It’s clear she’s infatuated with him, and obviously both Moriarty and Sherlock both know it. But what does Moriarty see? Sherlock is completely invested in his work, ignoring Molly as usual. He calls her date gay and returns to his work. Not something a good friend would do. He doesn’t appear to care at all about her feelings.
This explains why Molly wasn’t a target during Reichenbach. She didn’t appear to be very important to Sherlock, so Moriarty didn’t think of her as a potential target. Moriarty says his biggest weakness is that he’s “soooo changeable!”, but he didn’t take into account that Sherlock may have the exact same trait. No one, initially not even Sherlock, realized how important Molly could be. This left Molly able to help Sherlock outsmart Moriarty. Without Molly, Sherlock probably couldn’t have faked his death.
Also Andrew Scott is too cute for words.