Interestingly, there aren't any private boys schools in our town (although there is a girls school, and four co-ed private schools - not that we're in a position to go private). I did however consider a boys state secondary for ds, thinking they might have a better handle on the specific needs of boys.
In the event, it seemed too stereotypically boyish (in that it is very geared towards competitive sport, and you could smell the urinals from the corridor) and tbh I'm not sure that boys do have specific needs that can't be met in a co-ed environment. Fortunately boys don't seem to be have been demonised at his primary (or at least ds hasn't).
I don't have any daughters; perhaps I'd feel differently if I did. But I do think that if I had to choose between my child gaining 12 GCSEs at a single sex school, and 10 at a co-ed, then all else being equal I'd rather the latter. It just seems strange to segregate the sexes. I don't mix exclusively (or even predominantly) with women now, and didn't when I was a teenager.
ps wrt competitive sport, I don't have a problem with it at all, it's just not ds's thing (unless you're talking about motorsports, which aren't generally offered at schools!).