But that's never been what marriage has just been about in practice. In many cases, marriage was about money, sociopolitical alliance, or just comfort and companionship.Marriages are consummated by the action of sexual intercourse. It is tradition that a marriage isn't real until that happens. It is the teachings of the religion I belong to that a married couple must at minimum be open to having children or their marriage is invalid, as that is what the vocation of marriage is.
To make this sort of point, two 50-year-olds might marry: the clock has long since ticked past their ability to conceive, but they want to settle in together for the remainder of their lives. And yet the church will marry them!
That's not how it works.It's not a matter of justification, it's a statement of historical fact: Christians decided what the word marriage means. Now you want it to mean something different.
There is an overall concept of marriage in human understanding - and these are very broad. There are numerous words such as "marriage" that exist to describe this concept: these words fluctuate in spelling, pronunciation, and the precise details of concept that they are understood to represent. These concepts and words are held by individuals and, in sum, by general society. It is more accurate to say that at one time Christianity, as the dominant ideology of the day in certain places, had the greatest influence over the common understanding of what marriage (the concept) was in those places. But Christianity did not create the concept or the word, and it certainly does not "own" either. General society has always and still does determine what words mean.