In ASIP Sherlock is kind of clueless about flirting. Not reading into Molly's flirtations is a good example of that and you can't get anymore blatant than she dd. And yet when it came to John, he recognized immediately which says a few things. Mainly that

I’m just rewatching angelo’s because, I mean, how can you not and I just

that realisation

and he’s surprised but also flattered (which is not at all the kind of reaction we get with Molly and then later Irene) but also let’s John down so easy he’s so soft.

It’s hard to guess how he has responded to male flirtation in the past and how this might compare to how we see him with other instances in the show, but a) John was hella obvious (whether he is ‘aware’ of it or not) and b) Sherlock lives by this code of his that he is emotionally unavailable and married to his work and caring is a disadvantage but he’s still flattered and he still wants John to be a part of his life and work (and dear god just look at his face it kills me)