The local group here does have a fair number of women, so the problem is not universal.
I suspect that the idea of being on a bicycle by yourself in the middle of nowhere is on average more acceptable to men than to women. I haven't tried to conduct a poll to confirm that.
I suspect that if you have a lot of women in a local club, it's going to be a lot easier to get other women involved.
As far as I know, the local rando group here draws a fair number of people out of one of the regular bicycle clubs. If that club has a lot of active women, then you can expect that to carry through to the rando participation.
I don't fit into the normal club rider/racer/rando pattern that some people seem to expect. There are some of the local randonneurs that are friendly and encouraging anyway, while I get the impression some others would just as soon not have me around. I can imagine if you had a whole club like that second group, it would take a pretty thick-skinned individual to stick with it, male or female, if they didn't fit the mold.
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"be careful this rando stuff is addictive and dan's the 'pusher'."