A mixed sex toilet (as it is in this specific case) is one where the theatre has taken the “men” sign off the toilet door of the specifically designed male toilets and the “women” sign off the specifically designed female toilets. Then told patrons that all toilets are open to everyone.
So - men are able to walk into the toilets that were once designated female and use the cubicles.
In addition, women are expected to walk into the toilets that were once designated male - this involves them walking past the urinal (and thus seeing exposed tackle) to get to the cubicles there.
Uni-sex toilets specifically designed as uni-sex mean they are designed as single occupancy, self contained cubicles.
Mixed-sex toilets, in this and many instances now, are simply the existing sex-designated toilets opened up to both sexes. They are not being retrospectively outfitted as safe, self contained cubicles opening out onto a public space.
They have taken the “male” and “female” signs down and said go for it anywhere you want.