@MarieDeGournay
"I think 'because they are male' is a perfectly satisfactory reason for excluding transwomen from spaces designated for women. ... Statistical data about the risk... is an interesting and very serious topic, but it's not the only reason"
By stating that the biological category is a "perfectly satisfactory reason" on its own, you are confirming that for you, this isn't fundamentally a debate about statistical safety or physical threat. It is a debate about categorical definitions.
This validates the essay's point. She was trying to have a conversation based on risk and harm reduction, but she felt people were swapping her question for a question about definitions.
Your comment confirms that you aren't swapping the question maliciously - you believe the definition is the only answer needed, making the risk data irrelevant to your boundary.
"I don't see any 'societal cost' to this excluding men however they identify from women's toilets"
You don't see a societal cost because you are looking at it from the perspective of the majority, prioritizing tradition and privacy.
The author, however, is looking at the cost to that specific demographic - for example, the practical danger and distress a fully transitioned, older trans woman faces if forced by law to use a men's facility.
It comes down to a disagreement on whose distress carries more weight when crafting policy, and whether policies should be based on strict biological categories or practical risk assessments.