It's easy to tell if a poster is a man because they always start their posts with 'man here'
Except for the very many who don't...
Similarly, there's no way of telling from somebody's username what sex they are. People choose something they thing is funny, interesting to them or maybe the name of a real famous person whom they admire. That's one of the beauties of usernames, really: that you can be imaginative and choose something that is nothing like your own name. How boring would it be if all everybody were called LucyPerkinsOf28SchoolRoadSloughSL11AA - not to mention an extremely dangerous security risk?
Just as there are female users with obvious male usernames, there are probably also male users with female usernames - not necessarily because they're out and out perverts and sickos, but maybe just because they're, say, big fans of Taylor Swift/Tina Turner/Helen Mirren and just want to acknowledge this in their online presence. Are we to assume that women who call themselves something like BenedictCumberbatchsToothbrush are doing it because they're filthy deceitful perverts?
I think there's a bit of a parallel between 'men on MN' and all the MIL stories on here. I could tell you lots of stories about the things that my MIL has done over the years - but none of you would find them very juicy or exciting, as they're the kind of things that are routinely done by kind, loving, caring MILs - the sort that a great, great many people are lucky enough to have.
The name Mumsnet will have the instant effect of casually deterring a load of the most toxic men out there, whilst actively encouraging women, whether mums or not - which always has to be a very good thing; but we have to accept that there are still plenty of toxic people on MN: men and women; in fact, going on the user demographic, probably a significantly higher proportion of those people are actually women. Do we want misogynistic condescending, mansplaining Billy-Big-Balls arses on here? Obviously not. But we don't particularly want smug, bitchy mean-girls here either.