I don't see the point in even thinking about it, as we have no idea who anyone is on here, and no way of preventing men from joining if they want to.
What I would like to see is a rule that stops people from announcing that they are men. Their sex should make no difference to how they are treated, and there is no reason I can think of why it matters.
I have been in a few 'real life' groups which have started as all women, and when a man joins the dynamic shifts. Obviously that is mostly to do with personality - sometimes the man assumes that he should dominate - but more often it is that some women automatically defer to him, or want to protect the male ego. Not all in either case, but enough to make a difference. It happens across age groups - I've seen it happen in everything from knitting groups with largely older people, to student groups where the average age is 20.
We have a great opportunity online to just take people as we find them. Unlike in face to face situations, we don't know if someone is beautiful, we can't hear their accent or see whether they are wearing expensive clothes, how old they are, what they do/did for a living or if they are male or female. People can tell us these things, but what they say may or may not be true, and IMO none of it should matter. The Internet has many problems, but one of the many good things about it is that it is largely democratic.
So in short, I don't care if men join and it wouldn't matter if I did, but please let's not make a big deal about it or give them special status if they do join in?