The women I’ve spoken to (and the accounts I’ve read from others) all say roughly the same thing: It’s not being photographed that’s the problem. If everyone photographed them as part of the event, ie landscape shots of lots of people riding, it would be okay, but lots of men focus purely on the women, use zoom lenses to get closeups and are completely inconsiderate of their feelings. Being seen naked by a crowd and being photographed as one of a large group is one thing. Being zoomed in on relentlessly, knowing that closeups of your more intimate areas are being recorded for purely sexual reasons, is quite another.
Some women don’t mind that, but many are taking part at the limit of their body confidence. Blatant voyeurism is a hurdle many are not prepared to deal with.