Wow…that’s a long reply. I expected nothing less :)

I will try to respond to your points in order:

1. I think we can just agree to disagree on the importance of biological differences. I don’t think either of us will convince the other. I, personally, see it as a bigger difference than 2 years in age, or height or weight or allergies, or whatever differences you mentioned. I don’t know how to explain it besides saying that I believe that boys are different than girls.
2. I don’t know why you are “deeply suspicious” that I said that in some circumstances boys need to learn separate from girls. I don’t really understand why that is even controversial. The examples that you mentioned (building campfires or doing activities) are not what a group like the Boy Scouts is all about. It is what happens at the camp fire. It is similar to over night camp, which is often (not always) segregated by gender. What you learn from your peers is the real insight. And how one behaves around the opposite sex is just different than how they behave around their own gender. At least, it is that way with males. I don’t know how females act in groups of only females.
3. I think your assumptions in your third point are simply wrong. First, I think kids absolutely seek out same sex friendships for similar reasons as adults. I observe it with my boys every day. You also mention that with adults, it is during free time, whereas you create an analogy of Boy Scouts and adults jobs. I think this is simply wrong. Schools might be the better analogy to work, but Boy Scouts are a leisure activity done on the child’s free time.
4. I don’t know how much time you have spent studying the curriculum of the Boy Scouts. It sounds like you have done a lot. Far more than me. But every organized activity doesn’t have to solve every problem. It is perfectly reasonable for a male child to have one activity where they learn certain skills and another where they learn other skills in vastly different environments. I don’t understand why you feel that boys can’t have one place where they only interact with boys. Why does every social activity have to be co-ed?
5. “Let’s assume we shouldn’t separate children by gender unless there is a terribly compelling reason we should.” << I think we are talking past each other. You want the default for every scenario to be co-ed. I believe that it is is perfectly reasonable and very healthy to have certain live circumstances where boys just play with boys and girls just play with girls. That is a completely different statement than approving of what is done during those gender specific activities, which is where I think you have a problem. I am not saying that the girl activities should only be around cooking and sewing. And I am not saying that all activities should be gender specific. But don’t conflate the two distinct issues.