I’m not part of the “rational” foofaraw.
To summarize my views, (at least one of which will offend nearly every other participant in this thread):
Marriage is intended to support families. Although that is not its exclusive domain, it tends to be the oprational definition in the overwhelming majority of societies throughout history. It established both an identifiable unit in which children could be raised as well as establishing rights and responsibilities of inheritance and caretaking (both of children when younger and of parents when older).
Until 1960, the procreation of children was pretty much the default of marriage, (notwithstanding infertile couples or extraordinary efforts to prevent conception). In that year, the contraceptive pill was introduced, the ease of which use moved contraception from a special event based on condoms or timing to an action that could be carried out habitually.
Until 1978, conception was still a matter of either natural insemination or, at best, the collection of sperm for artificial insemination. In that year, however, in vitro fertilization permitted sperm and eggs from separate donors to be implanted either in the same mother after guaranteed fertilization or in a separate mother altogether.
At this point, neither procreation nor contraception are necessarily tied to heterosexual activity.
Coincident with these scientific developments, (and possibly related to them), social developments began reducing the cultural taboos against homosexuality.
As part of the lowering of those taboos, the movement emerged to permit homosexuals to be recognized to be entitled to all the same rights as any other persons–including to choose a life-long, (or extended period), mate with whom to share their life and property with all the rights and responsibilities accorded to or demanded of heterosexual couples who marry.
I think that enlarging the definition of marriage to include homosexual couples is a change. I see no reason to deny that it is a change from the concept of heterosexual marriage, whether monogamous or polygamous, as it clearly is a change in the language. I can find no reference prior to 1975 in which the word meant anytrhing other than some form of heterosexual union–and that usage was an explicit reference to a call to change the meaning to include homosexual unions.
However, given that the issues of procreation and contraception have already been separated from the necessity of heterosexual couplings for over thirty years and that, as I have already provided citations, there is a clear movement in which some portion of homosexual couples become parents while some portion of heterosexual couples eschew children, it seems clear to me that viewing marriage only in the context of sexual procreation has already been abandoned by society. Whether we view it as the basis of a nuclear child-rearing family or as simply the legal recognition of a couple’s commitment to share the rights and responsibilities of mutual support, it is time to recognize the expansion of the meaning of the word.