I came to say the same as many other replies. For older men, it doesn’t matter as much, they can simply create their own spaces, but for boys they really can’t, they are pushed into mixing in most situations. Boys are more boisterous, so need the organized outdoor spaces. They can’t get that male space from sleepovers like girls at that age do.
For another example, think of how a group of teenagers act on their own, now how does that change when you add an adult? It is obviously unhealthy for them to always be around an adult.