The Dutch have same-sex marriage, and both sides of the issue will be arguing whether the experiment has worked or not, and whether it has effected the straight population. This NRO article and this chart says that the out-of-wedlock births have been skyrocketing.
Here is a response from a hard-core gay activist. He says that correlation does not imply causation. It may well be true that same-sex coupling and out-of-wedlock births are both byproducts of anti-marriage trends.