I'm not a sociologist or a student of gender studies. I studied history back in the early 80s. Back then it was all political history. I loved it. My interest in toilets in India (and elsewhere too) comes from conversations with an old school friend who is the head of an Irish aid agency focusing on empowerment of girls.
'Mental gender' is a meaningless phrase unless you can say you 'feel like a man' or 'feel like a woman'.
Can you honestly say you feel like a man, and if you can say that, do you really mean that you feel like yourself?
How can you tell where 'feeling like you' ends and 'feeling like a man' begins?
What then is 'mental gender'?
Is it something to do with clothing or footwear choices or preferences for a particular personal appearance?
Gender is the system in which the adjectives 'masculine' or 'feminine' are applied to people and objects. The system is used to discriminate against women because it is the means by which masculine traits are marked out from feminine traits. After that is accomplished, a high value is assigned to masculine traits and feminine traits are denigrated.
When it comes to inanimate objects, feminine items and toys and clothing include jewellery, floral motifs, nail polish, high heels, impractical footwear for both women and girls, highly decorated clothing (buttons, ribbons, that serve no practical purpose), clothing without useful pockets, motifs like cute kittens, preoccupation with keeping clothes clean and in mint condition, all shades of the colour pink and pastels in general, dresses, tights, dolls houses, dolls, prams, cooking sets, long hair, makeup, sugar and spice and all things nice are considered feminine. Trucks, blocks, balls, bats, trousers, acceptance of jeans with holes and dirt, the colour blue, dinosaurs, wild animals, x-box games, shirts, ties, practical, well made, comfortable footwear, useful pockets, slugs and snails and puppy dogs' tails are all considered masculine, and there's lots more for both sexes...
For instance, women are said to have brains that are well suited to English Lit, sociology, and social history, to be nurturing, good at multi tasking, to like knitting, and to be nice, agreeable, pleasant and to take the high road and opt for dignity are considered characteristics women should aim for. Men are said to have scientific and mathematical and logical brains, to gravitate naturally towards political and military history and the hard humanities like economics, to enjoy competing, to be single minded and narrowly focused, not nurturing, etc, and they are encouraged never to let anyone put them down or get ahead of them. Because of the assumption that masculinity does not involve nurturing, you find there are very few changing stations in men's loos.
Also because of various gender-based assumptions, you find women being told they can't be astronauts or fire fighters or architects or mathematicians, and you find men being told they can't be nurses or early years teachers and boys are told they are sissies if they cry. With all of that goes the idea that men's jobs are worth higher pay than women's jobs.
Gender is quite the straitjacket, in other words, for both men and women. It also tends to keep women a bit poorer than men, in general.
'To drink the Kool Aid' means to accept some propaganda or party line or agenda that is being promoted.
'Birth gender' is a big fat heap of bullshit. You are born male or female. This designation refers to your anatomy and your chromosomes.
(There are very rare exceptions to this rule but they are completely irrelevant.)
You are then gradually assigned a gender. If you were born with a penis you will be treated as a boy. You will be given the benefit of the doubt when it comes to a good deal of behaviour. Your interests will be guided in the direction of 'masculine' things. You will be encouraged not to cry when you are sad. If you deviate from the set of appropriate interests considered ok for boys you risk being beaten up by other boys.
If you have a vagina you will be treated as a girl. You will be held to standards of behaviour that are different from those of boys, and encouraged to be interested in feminine topics. You will be allowed to cry but you will also be encouraged to mind the feelings of others.
Gender is just the means of preserving intact a cultural ideal of masculinity that is actually very toxic for boys and girls, men and women alike. It is toxic masculinity that makes effeminate men believe they do not belong in the men's loos. Toxic masculinity results in misogyny (hatred of women) - which is often just a fear of being considered less than manly, homophobia - again, often a fear of being seen as less than manly.
Men's loos should be safe places. They should be clean. Men should tolerate the presence of men in 'women's clothes' and with the appearance of women in men's loos. Men should tolerate the presence of gay men in men's loos. Parents should never have any reason to feel that sending a small boy into a men's loo or men's changing room would be dangerous.
The problem of acceptance of difference is one men need to solve.
Women should not have to accommodate men in women's places (and we are talking about far more than public loos here) because men have created problems for other men in their spaces.