I don't know, a few random guesses,
Though could be combo.
Maybe men use the internet more than women.
Maybe men 'used to use the internet more than women,
And because this site was founded by an older debate website,
It skewed it's starting ratio.
Maybe men and women have different interests or methods, (Generally speaking),
Whether because of culture or genetics.
Maybe more women use this site than is realized,
They just don't bring up that they are women.
Maybe more men are single,
And maybe single people use the internet more, or interact with strangers more online,
As opposed to something like Facebook, where one communicates with friends, family, coworkers, acquaintances.
I don't know what sex statistics are for other debate sites, forums, and what.