Mumsnet is a forum aimed at women.
Mainly mothers yes but there are plenty of non parenting talk topics and there is a big difference between childless women wanting to post on a female focused site than a man posting here, even if he is a parent.
As previously mentioned there are parenting sites that cater for men.
It mainly attracts women, but it was originally a parenting website for mothers and fathers - didn't Justine/MNHQ say that they'd considered the name Parentsnet, but thought it sounded less snappy? Either way, it has been confirmed time and again that it was never intended to be a women-only site.
Now, as you say, there are huge numbers (the vast majority) of non-parenting-related topics - most of them of just as much interest to both sexes.
Yes, there is always going to be the 'wrong kind' of man on here for nefarious reasons, but there are also a lot of women who troll and end up getting banned - should their sex give them a free pass? I rarely frequent the Sex topic, but on the odd occasion when I've glanced, the titles suggest a lot of very frank and leading discussions of all kinds - as you'd expect - presumably not all of them just from men?
I think the deeply unpleasant men who find their way here seriously skew people's perceptions into thinking that any male poster on here must be up to no good; somehow assuming that there couldn't possibly be numerous decent, genuine, ordinary men who don't feel the need to state their sex when discussing a parking issue, problem with a sick dog, CF, rising food prices, war in Ukraine, Line of Duty, Eurovision, the best places to visit in Canterbury - even a parenting issue(!) - and just value being part of a community of (mainly) wise, witty, interesting and engaging people - most of whom happen to be female, which tends to set the general tone and maybe attracts some of the more 'thinking' and less testosterone-fuelled men.
I can see parallels with MILs, when it comes to 'why are all these horrid perverted men on here?'. Let's face it: we all know what MILs are like, don't we? Take mine for example.... she's a lovely, kind, considerate lady - as I'm guessing that most are, including a great many posters on MN; but you'd never even guess that there could be such a thing as a treasured, much-loved MIL going on most of the times they're mentioned on here.