While I believe it is more common for men to pee in public than for women around here, of course those kinds of events would be the most likely place to witness women doing thier business outside and nobody diving a damn. However I tend to avoid huge crowds of people if alcohol is involved so I guess that explains why I've only ever seen men peeing in public and never women.
And maybe it's also my own bias. When I pass trough an alley or a stair case that smells of piss I might tend to assume that only guys left thier mark there while in reality women contributed to it as well.