You are complicating this too much. Just tell your kid that there are lots of types of families which can include famillies with one mommy and one daddy, single parents, grandparents, foster parents, as well as families with 2 mommies or 2 daddies. Then tell him that you and your spouse think that peope in all families should be treated the same, but the rules of the Boy Scouts do not agree with that and you don't believe that they treat everyone fairly. Explain that because you don't think that the Boy Scouts are fair to everyone, you and your spouse would prefer that he does not participate. I would also point out that these rules are made by many of the grown-ups who run the Boy Scouts, but it isn't necessarily a view shared by everyone in the organization - you don't want him to start thinking that his little friends are necessarily against gays.