Thanks for the replies. I'm glad I'm not the only one who feels the same way. Yes, you don't have to join Scouts, but what if you want to but can't because you happen to be gay? As Parky said, "being gay or black isn't [an opinion]" so people shouldn't be excluded because of something they have no control over.
@Spiritburner: You raise an important point about religion in scouting. I was already aware of the fact, although from my experience groups are so keen for new leaders that they wouldn't turn people away. I don't wish to defend my "beloved" Scouts on that issue - I would like to see the ruling changed so that you can join if you are not religious. In my opinion, no one should be excluded so I believe that the legislation should be changed slightly. (Having said that, Scouting does have a religious basis and as you said,"You don't have to join")
However, the no atheist rule in the UK does not excuse the Boy Scouts of America from banning people based on their sexuality. It is indefensible, in my opinion, to exclude people from an organisation because they happen to be gay.