I think there’s a big difference between my hypothetical “Christian group based around another common interest acting as an extra social lubricant” and @MalagaNights’ “Christian-only tennis clubs”. Honestly, I’d find both a bit strange, but the second a bit unsettling. Which suggests the distinction is partly semantic. Which is kind of what I was arguing - the word “allies” has become an issue.
If “schools shouldn't be segregating activities based on protected characteristics”, where are we in our argument for female-only sport? Or a Muslim prayer room? They’re more obviously justifiable, but if they’re in the same spectrum as my Christian group meeting for tennis lessons, then by condemning one, don’t we jeopardise the others?
I agree schools shouldn’t “discriminate”. There’s a difference between that and meeting partial needs (female tennis, prayer rooms), & allowing groups to coalesce around common interests & act freely (the - increasingly surreal the more I type it! - Christian tennis players).
The issue, to me, is when such a group makes a presumption of prejudice, encourages prejudice or tries to legitimise prejudice etc. “Allies” risks doing this, as would the implicit “No atheists allowed!” in your version of my (now, in my imagination, somehow, be-cassocked & racquet-wielding) group of predominantly Christian players.