The trend you mention is one that has been occurring in Western Europe for three decades or so, yet Norway (and most of Scandinavia) continually ranks high in both the prosperity and happiness of its citizens. Further, the trend has been occurring for longer than a cohesive gay rights movement, let alone gay marriage, has existed in most of these countries. Oddly enough, these are also some of the countries in the Western world that rank highest in communal thinking!You could argue that the fact that gay people are allowed to get married won't impinge on the ability of straight people to have babies, but you can't argue against the fact that the precedent alters the existing incentives so we're starting to reward people for making an emotional scene instead of motivating people to advance society in the most effective way possible. So in countries like Norway, where for many years marriage has been defined as an expression of love instead of a contract to foster an environment conducive to childrearing, you have many more cases of illegitimate parenting. Traditional marriage isn't meant to protect homophobic religious fundamentalist interests, it's meant to protect the interests of the future of society.
We can argue about the impact of easy contraception and no-fault divorce upon the psyche of the family, and there I would make an entirely different case, but it's ultimately a very different set of circumstances.