My DH had a circle of friends that was a small group of girls and boys throughout school and whom he has stayed in contact with as an adult. And I've known other men whose best friends were girls growing up. It's not out of the question that he'll stay tight with girls. But I've heard far more stories of kids who find their tribe in high school and college and aren't really friends with their middle school or elementary friends -- still friendly, but just 'grew apart' type stories. I agree about encouraging interest based groups -- that's the approach I try to take with mine.
