There are a number of possibilities. One of my male friends says (if I understand him correctly) that women are just distracting for men, and men would like some time off from being aroused under circumstances when they aren't supposed to show it. I'm sure he's accurate about himself, but he assumes he's typical of men, and I'm not sure he's right.
I've heard claims that groups of men behave differently if there's 10% or more women present. (Sorry, no cite.) If women have even moderate amounts of status, they may have a civilizing effect, and men could find that tiresome. The civilizing effect would vary according to culture-- it might be something like cursing is unwelcome, or at least must be apologized for. Or (specific example from the source I can't remember), if there are women associated with a fraternity (?), the men quit doing things like having indoor beach parties with huge piles of sand.
Really, I don't know, and there could be a number of reasons. It just seems like there's a tendency which shows up in many cultures for men to want men-only space.
Now, it is said we all here pride ourselves on our intelligence, rationality, and moral sense. It is also said, however, that we are a fiercely independent bunch, and that we can let this pride of ours get the better of us. There have also been comments that the live communities that appear at meetups provide much more positive interactions than what goes on on this site's discussions; this might merit further investigation.
My point is; we've done a lot of research on how to do proper ethical and metaethical calculations, and on how to achieve self-empowerment and deal with our own akrasia, which is awesome. We've also done some work on matters of gender equality, which is very positive as well. But I haven't seen us do anything about the basic details of human interaction, what one would call "politeness" and "basic human decency". And I think it might be useful if we started tackling these, for our own sakes, that of those who surround us, and that of easing our mission along, which is, as I understand it so far, to save the world (from existential risk (at the hands of (unfriendly and self-modifying) artificial intelligence))).
What inspired me to propose this post was a video I just saw from Hank Green of the famed and fabled vlogbrothers. I hold these two individuals in very high esteem, and I would expect many here to share my feelings about them, on account of their values and sensibilities largely overlapping with ours; namely the sense that intelligence, knowledge and curiosity are awesome, and that intellectuals ought to use their power to help improve themselves and the world around them.
Here it is; I hope you enjoy it