Al Mohler recently released his book We Cannot be Silent, a discussion of social changes that are inimical to Christian understanding of relationships.
In the article he canvassas a number of factors that have paved the way for the changes we now see: thus, none of them are new, merely a continuation of long tolerated trends.
The 'trend' as also aroused some academic interest, with James Wright asking similar questions in his editorial in Social Science Research (41) 2012.
Factors that come to my mind are: the now 'hyper-individualism' of Western society: unfortunately, a trend started for good by Martin Luther, but now we can all make our own standards. It used to be that society tolerated self-regualation as long as no one else was hurt. This has been washed away in same-sex 'marriage': the long term welfare or rights of children no longer count.