This thread is brilliant. I am distracting myself with it as DH is keeping me awake snoring and spreading out over the bed, so here are some of mine:
Women's uninterrupted sleep would be seen as very important, and everyone would understand it was entirely natural and unavoidable that, in sleep, women tend to spread out their limbs and snore loudly because they're in the most comfortable position. Men would be socialised to find snoring unmasculine, and would have been socialised to take up small amounts of space. After all, women have to go to work in the morning. It may be some men work too, but that is hardly as important - after all, men are much better adapted to be good at multitasking.
Men would feel hesitant about commenting on debates, especially in public, where they might be mocked. It would be commonplace for the media to run stories about men making false rape allegations and ruining women's lives. However, many 'ordinary' men (masculinists, but not that sort of masculinist!) would be outraged and declare it is 'taboo' to talk about the dangers of fale rape allegations because of all the extremist masculinists.
Women would seek out niche chat forums with predominantly male users and post made-up figures 'correcting' what the men had said, with no sources and in perfect confidence every man would immediately agree.
Women would become very angry at men questioning them or rejecting their viewpoints, and would use long words and pseudo-legalistic terminology to subtly remind the men of their lowly place.
Many other men would chip in to argue for the women.
The phrases 'not my Nigella' and 'NAWALT' would be commonplace on masculinist forums. In fact, approximately 20% of all men's forum traffic would in fact be contributed by women, and on threads started by women, it would be more like 80%. Such forums would still be seen as havens of remarkable masculinism by men, and they would repeatedly start threads celebrating this oasis of masculinist freedom.
Parents would refer to boys' genitals as 'smudge' or 'pickle' or 'woo woo'. Girls' genitals would be calls vulvas. Many adult women would not how to refer to the testes and penis accurately - the same word 'widgit' would be used for both, with a sizeable proportion of women unsure where to find the penis. Magazines would run stories about the 'testical orgasm' and men would write in to problem pages saying 'I am 30 years old and have never had an orgasm'. Agony uncles would write back telling them to relax their bodies and try to enjoy kissing.