The people who want to swim in a segregated session could go. To one of the single sex colleges (not sure if they have pools) or use one of the public leisure centres which offer this (eg Abbey).
It's the only college with a pool, or it used to be. And the college student body, the JCR, and administration are all in favour of the session.
I don't think it's wrong for a mixed sex college to have mixed sex leisure provision
Neither do the (confused) women who wrote the sign, nor the JCR, nor the mistress, nor the senior tutor. None of them are opposing the pool being mixed sex. Noone is lobbying to abolish all mixing of the sexes in Girton's pool.
Requesting one solitary hour a week for any women who want to swim in a same sex session is fine.
Declaring 'nonbinary' is welcome too is completely daft and expressly allows men to join in, undermining the entire premise.