The reality is that marriage does not now, nor has it ever been a reflection of anyone's morality. At least, not in the way Christians mean the word. Instead, individuals' choices about sex and marriage have been dictated by the cultures in which they lived. There is a clear and undeniable pattern in history: with technological, environmental, or philosophical innovation comes changing expectations of sex and marriage.
To begin with, we must recognize that the "Traditional Marriage" espoused by today's Christian leaders has never existed. One man, one woman, brought together in holy love, sexually celibate until marriage, sexually monogamous for life, raising their biological children together? Any sociologist or anthropologist worth their salt would scoff at the notion. This version of marriage is a myth, pure and simple.
Christianity began with the acceptance of polygyny, else why did the author of 1 Timothy pen this verse: "A deacon must be the husband of but one wife and must manage his children and his household well." (1 Timothy 3:12) If nobody else was allowed to have multiple wives, why mention it at all? Clearly, non-deacons were at least sometimes allowed to own more than one wife. There's also the uncomfortable fact that it wasn't until the 1500s that the Catholic Church finally outlawed all polygamy.
The bit about ownership is very important as well. Until the middle of the 20th century in Europe and America, marriage was about men owning wives. Although the language had softened by the Suffragist Movement, the reality was that there were virtually no legal protections for wives whose husbands mistreated them, and almost no avenues for women to seek or obtain a divorce. Furthermore, the social stigma of being a divorcee was so harsh that many women chose to remain in abusive, unhappy marriages as the best of all available options.
Herein lies the crux of the matter, and the fatal flaw in the Christian viewpoint. Any time we survey history for a significant change in marriage, we find that it is preceded by the addition of a new, better choice. Until the late 19th Century, virtually all marriages were matters of practicality, arranged by families for political, social, and economic gain. The idea of love in a marriage was actively frowned upon, so much so that men were often brought up before tribunals for the "crime" of being too soft on their wives, and showing too much affection. Marriages were arrangements, and while it was hoped that couples could find a degree of appropriate affection for both each other and their place in society, love was not a part of the equation.