Gay "marriage" is likely both an effect and a cause of family deterioration. A society that valued families would never even consider sanctioning gay "marriage" in the first place, so you have to figure the family was already on the downward slide in Scandinavia. However, once sanctioned, the gay "unions" cheapen the family even more in people's eyes, especially impressionable young people. They no longer see marriage as a special bonding of opposite sexes to create and nurture children, but just a pairing of any two (or perhaps more, I think Holland recently "married" a man and two women somehow) people who sexually desire one another. Who needs marriage for that? And if kids aren't an important part of marriage, and they can't be if gays can "marry", then why marry at all?
I would suggest an effect, but can't see it as a cause, because we are dealing with different groups of people. The issue is why have heterosexuals forsaken the traditional marriage/family. I see nothing that would indicate it is because another group (homosexuals) are getting married in 3 countries. This is also something that as you say began years before the gay marriage debate. So whatever the causes, the gay marriage issue may be an effect, if anything.
However, once sanctioned, the gay "unions" cheapen the family even more in people's eyes, especially impressionable young people. They no longer see marriage as a special bonding of opposite sexes to create and nurture children
Since this started long before gay marriages, and is rather prevalent in countries that have no same sex marriages, I think it is just speculative to assume that. If someone wants to raise a family and is heterosexual, I can't see them suddenly chucking it all because a handful of homosexuals got married. I believe there's a whole host of issues mitigating against the traditional family and marriage which have led to negative birth rates in much of Europe.
And if kids aren't an important part of marriage, and they can't be if gays can "marry", then why marry at all?
Again, logic is at issue here. If kids aren't important to a heterosexual couple, it has nothing to do with the fact that gays and lesbians can't have children. Hopefully you are not suggesting that kids have become less important to heterosexuals because homosexuals can't have any?
But this thread is not about gays and lesbians. It is about whether cohabitation should be an area for the state to control.