I think that depends on the circumstances, but happy to be corrected.
So: hospitals, schools, work places - have a captive audience, must provide single sex toilets.
Entertainment venues - eg a friend of mine has run a goth club night in a function room above a pub for two decades. Capacity maybe 150, & people have always used opposite sex toilets to check eyeliner in the mirrors in the ladies, or jump the queue in the gents. She also has quite a few trans regulars. Her understanding is that she just needs to clarify that both toilets are open to everyone, & if you buy a ticket to attend you're happy with that, & she's good to carry on.
(I agree with her fwiw - no one HAS to attend her club night, & tbh I'd be pretty chilled using mixed sex toilets there - it's a fabulous night & very friendly.)
It might get interesting re privately hosted events if the venue decides its toilets are now single sex, I suppose - can a promoter book the space & designate provision as they wish?
However, the big win is that orgs now CAN do single sex provision!
I don't think anyone was ever out to stop a few middle aged gents in corsets from doing their lippy in the good mirrors in the Ladies in between dancing to 'Temple of Love', IF & ONLY IF that's the deal at a private ticketed event, that's the organiser's view of what will keep their demographic happy, & it's clearly stated when you buy a ticket to attend.
On the flipside, presumably a lesbian group could now book the exact same space for a women (SC definition) only speed dating event. & refuse entry to any males whatsoever.
It seems to me that this is win win sensible legislation for EVERYONE. About bloody time.