Fr_Chuck is correct - the Boy Scouts of America is a private organization and as such can set their own rules with respect to whether they allow gays to be members. The Supreme Court ruled on this a few years ago - see:
CNN.com - Supreme Court says Boy Scouts can bar gay troop leaders - June 28, 2000
The BSA also won't let atheists join. I am not defending the BSA position on this, just their right as a private organization to have a position. Personally I think the Girl Scouts have a better policy in this than do the Boy Scouts - the Girl Scouts do not have a rule forbidding gay or lesbian members.