Perhaps rather than skimming you should try reading the thread, where this is properly addressed.
To summarise:
In a social context where it is accepted and respected that toilets (or whatever single sex spaces we are talking about) are single sex rather than anything to do with gender, most people, including trans people, will respect that convention. So women will tend to assume a transman in their space is exactly that, a transman, aka a woman.
In a social context where it is accepted and respected that toilets (or whatever single sex spaces we are talking about) are single sex rather than anything to do with gender, women will feel empowered and supported to challenge someone they believe may in fact be the opposite sex.
This means a few transmen may be challenged, which may cause embarrassment and discomfort on both sides. Apologies will be made. Similar to what happens if you push open a cubicle and find someone is already there...it's not supposed to happen and we're all a bit embarrassed but we all realise it's a genuine error.
It also means a few men of nefarious intent may (a) pass as possibly transmen, (b) lie if challenged, and (c) not do anything to reveal themselves by their behaviour.
While this means it's true the social contract (or even a law) cannot be 100% guaranteed and broadly relies on self compliance coupled with risk of shame and humiliation if caught breaking it, that's no different to any other law/social contract. We can't police everything all the time and we can't guarantee everyone who does transgress is caught. Nevertheless, we still consider it worthwhile having legal or social rules like "don't steal", or "don't shit in the street"
in brief, no we can't guarantee 100% compliance, but that's not different to any other legal or social rule, and most of us think the advantages when it does work are worth keeping even accepting the occasional breach.