It certainly feels like the closest Nolan has come to an out-and-out non-heteronormative relationship between main characters.
With the way
protagonist and Neil talk to each other at the end of the film, with protagonist basically on the verge of tears during their goodbye
, I certainly chose to read it that way.
Spoiler for Tenet.
I think Nolan said that as he was making the movie, he and JDW realised that the real love story was Neil + Protag. I don't think he intended it to be sexual (that's his third movie in a row where character don't even try to have sex btw), but he eventually got that the two emotional heart of the film are Neil+Protag and Kat+son, and that Kat + Protag was much less important than these two.
Anyway, I'm waiting for the sequel where Keri Russell pops from nowhere to say she's had a relationship with the protagonist long before and disappear right after.