I don't ride with either of the local clubs because I'm sure I would just hold them up, and it's a constant worry for me when I try to do different rides. One of the reasons I started audaxing was because the challenge was totally against myself, and I wouldn't affect anyone else's enjoyment of the event (on my first audax ride I was never going to finish in time, and distinctly remember ringing up and persuading the org to not hang around and wait for me as I didn't want to inconvenience him). In my experience of meeting and riding with different people I would say there are a lot more men out there who are in a similar situation who would benefit from a 'Breeze' style ride as much as women with a similar outlook, and this might even dilute the stereotypical 'testosterone fuelled' male cyclist image. As it is, though, with women vastly under-represented in cycling numbers, it makes far more sense on concentrating on that challenge first.
There are obviously other barriers that seem to prevent women from pursuing the same loner style of cycling I followed otherwise the number of female cyclists overall would be closer to that of men. Some of these may be psychological, but I would suggest many of them based around the societal attitudes* towards things such as women exercising, the way women should look, and the way too many men feel they are allowed to behave around women. That's where, as an outsider to the rides, I see the real benefit of female only group rides, as a space for unconfident female cyclists to get used to regular cycling in an environment that is (or is perceived by the potential new rider to be) more supportive of female cyclists.
Coming back to the OP, do I think they are being sexist. Not really, assuming that potential female Sky ride leaders are not being pushed to run Breeze rides instead. As has been pointed out, ride leaders for the compensated rides is open to male and female organisers equally (regardless of the uptake, the remuneration goes to the man/woman organising). On the other hand, I would very much like Breeze rides to be funded better and encouraged more, and that could be sorted completely independently to the mixed rides.
* As an engineer - venturing into sociology is something I do very warily