Obviously I am not proposing that men who identify as trans should be in women's spaces. But what are the reasons to resist third spaces? I'm not looking for a debate - there are abviously reasons for third spaces - I'm just trying to consider whether there may be a fair few good reasons to resist them.
(1) It is tough enough on small businesses that they (rightfully) have to provide third spaces to people (disabled people) who literally cannot access single sex spaces. Giving them further obligations based on feelings not physical need is an unreasonable requirement. It is also tough on the taxpayer when government has to provide third spaces. I can imagine some small business owners becoming transphobic when they realise they have to spend thousands of pounds on building works for a "problem" that has never arisen in their workplace and which counters their legally held views that TW are men, end of.
(2) Some women with trans identities might feel obliged to use third spaces and thus find themselves at greater risk from men that they would have been had they just used women's spaces. The very existence of third spaces might increase the risk of harm to some women.
(3) Trans women don't get validation from third spaces so they are 100% unwanted by the main people who theoretically need them.
(4) We need to bring people together as much as possible - not divide them. Single sex spaces make huge amounts of sense for various reasons. Providing one accessible toilet makes sense (whereas making every single male or female toilet accessible makes much less sense as it would cost too much and take up too much room and would be a disproportionate way of providing for the disabled). But dividing people even further by creating more spaces - when the truth is we are all simply men or women at the end of the day - makes no sense.
(5) When a group ignores boundaries and the truth and courteous debate and instead seeks to bully and threaten and make people feel uncomfortable then one has a moral obligation to resist giving them an inch, for to give an inch is to reward their behaviour, which can only risk encouraging them and people like them. It reminds me of the slogan "we don't negotiate with terrorists".
(6) The third spaces would risk becoming a target from those men who do genuinely hate trans people. Trans people are at risk from transphobic violence (I am not sure whether they are significantly at risk in the UK, I doubt it, but the risk is there) wherever they are. Walking down the street, in the appropriate single sex spacecs for their sexed bodies, in opposite sex spaces, in a pub, in a shopping centre - they could be attacked out of the blue anywhere. But to actually create a space which violent transphobes know to go to if they want a victim could create more harm than good.