I agree that it is impossible to change sex and that gender is a social construct. However I would not challenge a male-appearing person in a female toilet as it would be entirely possible for that person to be a biologically female person who is choosing to present themselves according to masculine styles but who has every right to use the female facilities and may well be vulnerable to attack/verbal abuse/corrective rape by transphobic men if they use the male facilities and are detected as being biologically female.
I also think it's a bit silly for small places like cafés that only have space for 2 cubicles to label one with a female sign and one with a male sign.
The problem is that we are trying to formulate one set of rules which covers a vast number of different sizes and types of facilities.
Somewhere like a motorway service station or shopping centre I think everyone would be safer and more comfortable if all facilities were unisex with floor-to-ceiling doors and no open urinals, and enough larger cubicles a with mirror and sink of their own that anyone who needed to get changed or wash out a moon cup could do so. And enough staff on duty that if a person who pees standing up wees on a seat then it is cleaned immediately. And ideally several alarm-points so that if anyone was behaving in a creepy or inappropriate way they would be removed - but there would generally be no problems as traffic would be high and generally creepy pervs don't be creepy when there are lots of people about. There would be ways to keep these unisex facilities safe for everyone, like the unisex changing facilities at many swimming pools.
However those same principles can't be transferred to a pub where the available space is two small rooms with a pair of cubicles in the ladies' and 2 urinals and a cubicle in the mens'.
Perhaps the solution is that all businesses and services to be given notice that by X date in the future (2 or 3 years to allow for budgeting) they must
(1) convert all loos and changing facilities to being unisex with individual private lockable cubicles (ban communal changing areas and places with just a curtain)
(2) have in place a proper policy and action plan in which staff are fully trained and regularly drilled for ensuring that anyone of any gender presentation behaving in a pervy, inappropriate or antisocial manner is removed, and anyone being followed/controlled against their will has an easy-to-access escape method clearly publicised so that the previous method of escaping via the ladies' isn't missed.
The consequences would probably be that a vast number of small businesses would stop offering toilet facilities at all because it would be unviable for them to properly manage a small unisex facility.
It's not the people who #justwanttopee who are the issue. It is that there are way greater numbers of predatory men who will abuse any opportunity they have to dominate women than there are trans people who #justwanttopee. There is a willfull blindness, probably from naivety and inexperience, that setting up rules that make everything lovely for the harmless non-predator who #justwantstopee is also making things far too easy for predators and those who get sexual pleasure from public lavatory situations.