I'm not blanketly against it -- I think it can work for some people.
The most compelling reasons I saw for it were for shy girls who were being shouted down by more outgoing boys, and not participating in class even though they did know the answers. They could really benefit from an all-girl environment.
This comes up in my non-profit organization -- it's women-only, and some of the women feel strongly about not allowing men to certain events, because they feel like men always take over everything.
For myself, throughout my life, my social circle included boys. That was not only in terms of recess or whatever -- I'd do school projects with boys, and frequently competed with boys in a way that I found fun, the boys in question found fun, and girls tended to shrink away from.
Sozlet would also absolutely freak if she could no longer be in classes with boys. Several of her best friends are boys, and she regularly has moments when she's just kind of over girls and their drama and needs to hang out with boys. This happens during the school day too.
Plus, especially at her age, it's really hard to "mingle" in a non-datey/ flirty way without the incidental contact at school. If you have to do a project with a guy, of course you're going to talk to him -- if you never see him during the school day, it's much more awkward to seek him out and chat during non-school hours.
And she's thus far resolutely non-datey -- she's inoculated from teasing about hanging out with a "boyfriend" because she has a bunch
of friends who are boys, rather than one specific one that becomes a target. That works both ways -- the boys don't worry about hanging out with her, either.
So, for me, and for her, I think co-ed is best.
That doesn't mean it's best for everyone.