I recently came up with an idea concerning homosexual marriage. The biggest draw back is that it will take some time.
Here it is:
Take thirty liberal homosexual couples (20 to 25 years old) and drop them on a completely stocked, isolated island.
Also, take thirty conservative heterosexual couples (20 to 25 years old) and drop them on a separate completely stocked, isolated island.
Leave both islands totally and completely isolated. At the end of 80 years go to each island and ask the residents which group would be more beneficial to society. If the homosexuals give the best answer, homosexual marriage becomes legal. If the heterosexuals give the best answer, normal marriage remains.