If you look at the development of young people period, it's much slower than it used to be. Adolescence is extended by high school+college+graduate school. You see more kids now coming back and moving in with their parents where a few decades ago the expectation would be to go out on your own and make your way in the world once you reached driving age or so. At least that's how my parents tell the tale. The "by the time I was thirteen I had a wife, a horse, and a job" days seem to be gone for good.
Every Lifetime Original or Tyler Perry movie is full of guidelines on what makes a "real man" or "real woman" for that matter. "Real men don't cry." "Real men take responsibility." "Real women have curves." "Real women don't have curves." The point is that we have no idea what it means to be a man or a woman. Everyone has a different idea and the process of getting to manhood or womanhood is largely unsupervised. Kids figure it out in isolation or with their equally clueless peers. Signle gender environments provide a place where kids can talk openly and honestly about their identity in an academic setting.
Perhaps the differences in academic performance are less about the "gender distraction theory" and more about the social and emotional health differences in students in single gender environments compared to those that are in coed schools.
For young boys in particular, this quest to become a man is ill defined and has no clear beginning or end. Most fathers don't take their sons into the woods to complete some task that makes them a man. There's no standardized coming of age ritual anymore. That's what makes this so confusing. That and the relatively new idea that being confused about life is ok and that you can take as long as you need to figure it out. Single gender schools create a unique place where kids can figure their identity out. It's the inability to discuss gender in a judgement free environment that causes what many people would call a male culture of risk taking and violence. I don't buy completely into the argument but it is fascinating to look into.
One of my favorite books on the topic is Dr. Michael Kimmel's Guyland. If you read it and disagree with his argument, Dr. Christina Sommers' The War Against Boys offers a few counterpoints.
Kevin Malonson on Single Gender Academics - Learning Curve
Kevin Malonson stops by the Dallas Education Podcast
Single Gender Schools Research