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.
Gay "marriage" is clearly an effect because it's hard to imagine a society that valued families sanctioning gay "marriage". But once sanctioned, it does additional harm.
####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 think you're correct as far as older adults are concerned. I don't think the addition of gay "marriage" in a society would harm the marriage of an established married couple. It's teens and younger people who are likely to be suffer the effects.
####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?####
No, but the sanctioning of gay "marriage" means that creating and nurturing children is no longer of any special relevance to marriage. It's just about sexual desire, not building the future of society. It's just about each "partner's" gratification, not about laying the groundwork for the raising of the next generation. If marriage is no longer about kids and the well-being of the next generation, then why bother with it? The only difference between being married and not married is that the former requires a few more obligations, and who needs that in a "me first" society? That's the attitude conveyed to kids by gay "marriage".
It may seem ironic to some that liberals dismiss marriage as not being important and then demand that gays be extended the right to "marry". But it isn't unusual if you understand liberals. They're merely covering all the bases in their war against the family.