Maybe both the name bits on the tarmac were callbacks to other scenes.

Obviously the whole “William Sherlock Scott Holmes. That’s the whole of it. If you’re looking for baby names.” is a callback to John’s jealousy during ASIB between Sherlock and Irene.

But perhaps when Sherlock says “Sherlock is actually a girl’s name”, it’s not just him trying to see John smile. Maybe it’s him trying to tell John that he was wrong from the start.

Because in ASIP, the very first thing that we see Sherlock get wrong is the gender of John’s sibling because the name Harry throws him off. He automatically assumes that John has a brother, but he finds out he’s wrong later when John corrects him and says that Harry is his sister.

So maybe when Sherlock’s first name-callback doesn’t work, his last comment isn’t just a joke, but rather a second attempt to hint to John how he feels, how he’s always felt.

In Sherlock’s own words, he says “John, there’s something I should say, I’ve meant to say always but never have… Since it’s unlikely we’ll never meet again, I might as well say it now…”

This is followed by “Sherlock is actually a girl’s name.”

Names in this show are associated with trust and love, we’ve been shown that various times (John’s jealousy+giving away his middle name, Sherlock finding out John’s middle name+realizing he loves him, Mary not giving John her real name+John not being able to fully trust her). We know what Sherlock is talking about. You can see it in John’s face too; he’s expecting Sherlock to say it, which is why he is both relieved and disappointed that Sherlock doesn’t say it (he wanted to hear it but wasn’t ready to hear it). And it’s been established now that when it comes to names, Sherlock is usually wrong. This thread continues throughout, even in cases where it doesn’t necessarily line up with the trust/love theme (Sherlock messed up the gender applied to Harry’s name, didn’t know John’s middle name, couldn’t remember Greg’s name, Moriarty/Richard Brook situation framed him as wrong, said the wrong name when Magnussen was being held at gunpoint, never found out Mary’s name).

We’re supposed to associate names with Sherlock being wrong, and also with love/trust, depending on the context. And what’s the topic under discussion on the tarmac? Love.

Sherlock is not making a joke. He’s telling John “I was wrong about how I felt from the start. I said I wasn’t interested and that I didn’t care, but I was wrong. I do care, I am interested, I am jealous. I do love you, John.”

Because Sherlock thinks it could work. He doesn’t mean naming the baby after him. He means him and John.