I don't think it's a sexist thing to simply state that 'gender shouldn't matter'. That is a valid comment all things being equal and hopefully that is where we all end up.It's remarkable that the suggestion gender should not matter is itself considered sexism or a marker for it. I'm more inclined to think the characterization of men in this thread is the most sexist thing I read. It's a forum that allows for it, not unlike a men's only club would in reverse. Yes, sexism exists. Abundantly both ways.
I would inherently prefer a male instructor, because they are more likely to be like me and I'm more likely to relate. l would never allow myself to take that poison pill, because it simply perpetuates the division. It's no more valid for a female to generically predetermine other females more capable.
You pick. If you're okay with segregation, you're okay with it. If you're not, you don't get to proclaim your indulgence to be appropriate.
However my issue with your viewpoint is that in this specific case it ignores any historical or contextual power imbalance or gender bias. Both of which in my experience have existed in sailing.
Heard of Kay Cottee? She was the first ever female member of my yacht club 30 years ago (and she had to sail solo around the world before she got membership, I just had to get my mate Dave to second me). 30 years is not nearly long enough ago for the yacht club I grew up in to have it's 'first female member' in my books.
So I think it is perfectly understandable and reasonable that some females, entering a historically male dominated sport might find it easier learning with a female instructor.
Whats the big deal with that?