I suppose one could argue that masculinity has always been growing rigid (*cough*), just as the middle class has always been rising. Certainly it's an argument I've seen made by historians for every western period from the middle ages to the early twentieth century, and I imagine classicists and history-of-yesterday folks make it too. But it does seem to me (despite being very much Not An Expert) that something big happened between about 1910 and 1920, so that discourses of male friendship that were possible before then (e.g. fictions like Holmes) ceased to be available.

I've seen people claim the Wilde trial as a turning point, but there's still a lot of male-male homosocial/homoromantic writing (e.g. Raffles) being broadly produced and consumed after that, whereas postwar with something like Ackerley's Prisoners of War there's a definite feeling that something that might've passed for male friendship a decade or two earlier is now specifically being represented as queer.

/speculating on subjects in which I am Not An Expert.