It’s always been about sex. Sex outside of marriage was technically illicit in Christian society, and marriage implied consent. For example, there was no such concept as marital rape until very recently rape had an entirely different definition from today. Also, virginity was considered a woman’s property (or her male relatives’ property) in most cultures, which had a lot to do with the old definition of rape. This means that the concepts of sex and property are not strictly separated in regards to marriage, and are actually quite closely intertwined. Christian ideas about marriage were kind of revolutionary in that there was a concept of mutual ownership of sexuality in marriage (e.g. a woman had as much right to demand marital fidelity as a man), rather than the simple chattel status of wives most common in other parts of the world. The modern definitions of marriage and rape have actually reversed what was the norm in most of the non-Christian world, and placed ownership of all heterosexual sexuality entirely in the hands of women regardless of marital status. Accordingly, it has been devalued a great deal, just as one might expect a car company to become essentially worthless quite rapidly if it were entirely owned and controlled by women.