Do I understand you to say then, that marriage has been (continually) re-defined since the 60's? If so, how has it been "re-defined as opposed to modified?
Marriage was to have been one-man and one-woman for a lifetime except for extenuating circumstances. It was also to have been the reasonable and appropriate realm of sexual relations.
Since the '60s we have dumped marriage as the reasonable and appropriate realm of sexual relations. Sex is separated from marriage now.
Then we dumped the lifetime. Serial monogamy might be preferred by some, but lifetime monogamy came under attack. Marriage is for the moment now.
We are in the process of dumping the "man and woman" part.
Expect the exclusivity arrangement to fall next. Don't even get me started on interspecial marriage.
Shalom.