How do I combat the "women need safe spaces" argument?

There's a tool that I got taught as a Business Analyst. I already knew about it, because I already use it everywhere in life, but it was helpful for someone else to put a nice short label to it. It's called "The 5 WHYs" The idea is that you start with an initial problem, and ask someone "Why?" When they give you an explanation, you pick the key point out of that explanation, and ask "Why?" You repeat this 5 times. By the time you've got to the 5th "Why?" (or possibly earlier), you'll have learned the true root cause of the problem.

If you apply the 5 Whys to people's personal opinions (in any field), it can be quite revealing. If you apply the 5 Whys to these women who claim they need safe spaces that exclude transgender women, you'll eventually get to the root cause of their objection, and it will be that transgender women are not really women. It's as simple, and as complex, as that.

However these women present their arguments on the surface, using logic or rationality or reason, the underlying justification is a rejection of transgender people. You can't argue against that. It's not rational or reasonable or logical. You can present arguments, but your correspondents won't accept them. You can't use logic against emotion: it's like using a sword to fight a wave of water. It's the wrong tool for the job.

The argument has been doing the rounds in the UK for a while now, I think Shaun's video on the subject still holds up for this particular argument. Algernon's comment is a valuable insight but unfortunately you might find yourself in a situation where somebody clearly isn't being cooperative or is trying to publicly fearmonger rather than engage in an actual discussion.

I don't know how comfortable you are with the idea of engaging with bad faith actors, but if you are you'd probably come out best pointing out that the practical implications of these demands would limit access to Safe Spaces for cis women or discourage them from seeking them out at all, because it would require highly invasive "checks" to limit entry. They would effectively be throwing women as a whole under the bus just to assuage their personal fears.

To go back to Shaun's points there are a couple of reasons as to why creating spaces exclusive to cis women on the basis of their biological sex you run into some really awkward problems:

• If this means regardless of genitals then you now have to enforce a check on medical records to confirm if somebody was indeed assigned female at birth. I don't know what's worse about that: the violation of privacy or the fact that it'd require all domestic abuse shelters to be equipped with the necessary tools like it's a security checkpoint.
• If this means depending on genitals then you have to deal with the possible implications for intersex people. And if that gets hand-waved away as being too small a group you could very much raise the question if this means we're assuming that any kind of medical or physiological issue makes somebody politically disposable as long as the condition is rare enough. (the answer for a lot of these people is yes, but they'd rather not say that out loud)
• Additionally: does this mean they are proposing all Safe Spaces for women have their access restricted via invasive body searches to confirm the state of their sexual organs? Is that really what we want for, say, a Safe Space for victims of sexual assault?
• Lastly, and this is usually where you end up, there seems to be a supposition that trans women can just be told from cis women by looking at them. Meaning that access to Safe Spaces would now be restricted based on some arbitrary standard of physical appearance of femininity. So effectively if you're a rape victim and you look too "mannish" for whoever controls access at the Safe Space you'd be out of luck.

A lot of anti-trans arguments seem like recycled anti-gay arguments. I remember similar arguments against gay men and women predators sharing bathrooms with straight people, though I don't remember any catchy counter-arguments besides the observation that present gay acceptance hasn't lead to an epidemic of cases like that.

Genuine question: have there been cases of sexual assault where the victim is a female cis female-bodied person and the aggressor was a female trans male-bodied person?