I loved being in a single sex sixth form, but I did lots of extracurricular stuff with boys and always had done. I think single sex education is possibly better when you're younger - obviously I only know one side of the story, but it made the whole puberty thing a lot easier as everyone was in the same situations. As for sixth form, I don't know. My whole year became incredibly close and I don't think that would've happened if there had been guys.
While a co-ed environment prepares you for the "real world" in that there are (shock!) two genders, it's school, and that isn't the real world.It's important to have a mixed friendship group, but that can happen very easily out of school.
Also, practically, how many boys would go to an all-girls sixth form, at least for the first few years?
Sorry for the anecdote-yness!