" Meanwhile in the UK, they've also allowed gays and their membership is growing so fast they have to limit membership due to a shortage of adult leaders. Post hoc. "

Meanwhile, in Canada:
http://www.scoutscan.com/issues/membershipstats.html

I don't have statistics handy for the UK Scout Association. It would be interesting to see them, especially if they're broken out by section.

(Note that I'm not claiming the stats say that allowing homosexuals results in a decline in membership. But it clearly doesn't turn an existing decline around.)