What do they think should happen to the female people that are excluded from any space so that male people can take their preferred choice from all the spaces?
The female people with autism, dementia, other disabilities?
The female people who have been sexually assaulted, domestically abused, traumatised?
The female people who are orthodox Muslim, Jewish, Jehovah's Witness, Roma Travellers?
What are we going to do with those females from this group who will not be able to access medical care or stay on hospital wards where male people are? Or will stay in life-threatening DA situations because they can't access a refuge? (Mention the MNetter who lives rough in a tent currently and has for months because no single sex refuge). Or will be excluded from society and spaces like gyms, swimming, women's groups when they're mixed sex?
If they don't get the issues or they try to dismiss them then they have not fully 'educated' themselves or looked into the issues, the needs, the situations of these female people they're so readily dismissing, and perhaps need to do some research before they embrace being really rather ableist, racist, xenophobic.
I usually find with dedicated political lobbyists that they will do anything at all to avoid acknowledging that these women exist: once or twice with back against the wall there's been the flat out attempt to say that there's only a few of them (there are a whole lot more of them than there are male people wishing to be in female spaces) and eventually will admit that excluding these vulnerable, marginalised females is acceptable collateral damage for the greater happiness of men. At which point really the sexism and many other isms are so overt that using words like 'kindness', 'inclusion', 'diversity', 'respect', 'acceptance' and 'intersectionality' are just ridiculous. No, however uncomfortable it may be to admit, it's just flat out male supremacism to extend those values to male people while actively removing those provisions from females. The unequal standards are indefensible.
Ask about the cotton ceiling: should females have to learn to 'get used to' sex with people with penises, which is essentially about females having no right to enjoy sex or make their choices and all about providing service to a male person despite it being no more pleasant than something they can 'cope with'? And show them the quotes. This is straight from well known activists who are public figures. Suggest they look up some incel material and do a bit of compare and contrast: what do they notice? What does this attitude express about respect for females, empathy for or healthy relationship values for another person? Consent? Is this the kind of sexual relationship they see as a good thing for both partners equally? Or do they feel that being born female imbues some kind of service requirement and accepting lesser in life for the benefit of those more valuable and important than them?
Are they personally keen to embrace that service? Should their choice mean all women are forced to embrace it?
This is more for you and your internal tool kit than direct suggestions: but it's about helping them realise, they're speaking from a position of ignorance and unconscious sexism. They just don't know yet what they don't know.