So, the main gist of what we've got so far,

There isn't/wasn't an imbalance.

More women are involved in playing/singing due to encouragement than they used to be.

Women aren't as good as men generally, why should it be different in singing?

Women are/were culturally trained to be more reserved than men so don't push themseleves forward as easily.

Women more prolific in the singing scene than the pub session scene. Poss. due to a social statement surrounding women going into pubs alone.

Women tend to be in a supportive role, doing the food/posters etc..

Women have a different repertoire which doesn't fit with the folk club scene.

The 'big' women in folk are of the sexy pop variety, not real folk singers.

Women don't/didn't play guitar as commonly as men so had to rely on accompaniment (usually by a man) or appear unaccompanied.

Women had to stay home and look after the house and kids.

Women's voices sound worse than men's when singing badly.

A lot of clubs were connected to colleges/universities which had a higher percentage of male students.

More men than women want to go out and sing in the evenings.

Any more for any more? what about the 'tone' of the evenings? were there male dominated conversations? political connotations which women weren't involved/interested in