I went to an all-boys school: I liked it and did very well there, but I suspect single-sex schools were more common then than they are now. I think the temptation to show off to the opposite sex all the time can prove too much of a distraction in mixed schools, and if you do encounter relationship problems with your girlfriend, it's probably better if it can't spill over into the classroom.
What's wrong with realistic experiences like learning to interact with the opposite gender when young and going to school? Or is it better to shove them into the grown up world with no previous experience?
Hey Wait! Teenagers Learn???
There's an age old belief in the world of private education that girls learn better in single sex schools and boys do better in co-ed schools. Go figure.