I have no issue with men posting and chatting away on mumsnet with women. Often you don't know, and can't tell, they are men. But there really is a different type who do come on to threads, or boards, and demand attention. I'm surprised, Lord, that you haven't seen these threads as they are really very common!
The two types which are most infuriating are the men who start a thread on this Feminist board claiming to want us to 'educated him but that always, without fail, goes on about how we are all men-haters, how we should focus on the male suicide rate, how the women in his life are not victims, in fact his boss is female and there are mostly women in his office, then tells us how only 6% of women identify as a feminist. Often these threads descend into the man calling all the women bitches and getting banned and the thread gets pulled, so many that's why you don't see them. They also tend to target the boards at specific times.
The other one is the man who comes on to say about how he does all the work, earning, childcare, and housework, but his wife still won't have sex with him.
These two types are the ones I just can't be arsed with, and I'd like them to just fuck off and piss all over some other website. They happen far too regularly and I think they are mostly bullshit, sorry.
The "poor menz" is generally done when women are talking about a harmful women's issue (often one caused by men, or patriarchal culture) and posters start WATM!-ing all over the place. It is usually said in exasperation, given how many times women's issues are overlooked or derailed by men/men's issues. I do wish men would stop telling women to spread their emotional labour around wider so they can also sort out men's problems, when men (as a class) have all the power, influence, and money to do these things for themselves and fix things/provide services, if only they chose to.
On the other hand, I did enjoy the thread asking men their views on are TWAW. It was really nice to see men just talking normally about their views on that, and I was impressed on how well-informed many seemed to be on the issues (hope that doesn't sound patronising, I don't mean it to).
So, to summarise, no issues with men on mumsnet, just don't make it all about you. If you need help, post, sure, but some women are too busy focusing on helping women and that is perfectly acceptable.