Anyone who has looked at what marriage has been down through history will know that the variety is extremely wide--including homosexual and group marriages. It's good to note that nothing in the marriage ceremony or in law states that procreation is a requirement. Marriage in our culture is about love and committment. People used the same arguments against women having equal rights since women were (obviously) not men, the same was used against interracial marriage, inter-faith marriage, and plural marriage. All of those arguments did not prevail then and they won't prevail now because they are irrelevant. Gay marriages in other countries and in one American state have done no damage to the institution of marriage just as the marriage of gay people during the first 14 centuries of Christianity did no damage.
Indeed, definitions do change over time, usually as new relevant knowledge is validated to be accurate.
Nevertheless, some restrictions are simply irrelevant to the definition.
For example, the time-honored definition of marriage is between
a man and a woman as husband and wife.
Race definitively matters not, yet real bias may have inapproprately restricted marriage racially in the past. Likewise, inter-cultural/national/tribal "taboos" may also have restricted marriage, even though the prospective participants, a man and a woman desiring to unite as husband and wife, fit the required definition for marriage. We would argue that these definitively inappropriate restrictions were based on bias.
It is important in our more rational, enlightened here-and-now that we not impose such restrictions that are definitively inappropriate. Such irrational
restrictions are forms of right-wing extremism that does not reflect accurately with respect to the defined subject matter.
On the other hand, it is also important that we do not equally error with left-wing extremism via irrational
inclusion that is definitively inappropriate. Thus gay/lesbian marriage is
definitively unacceptable. So is polygamy, by time-honored and still-respected definition.
It is thus vitally important with respect to neuropsychological evolution in the name of rational common sense that we remain
centered with the historic and modern vast majority
definitively and therefore respect the minimum qualification for marriage being between a man and a woman as husband and wife.
That some places now and in the past have behaved infrequently inappropriately erroneously extreme, either left-wing erroneously extreme or right-wing erroneously extreme, with respect to the definition of marriage, does not excuse that definitive wrong-doing nor in
any way justify a definition revision.
Brother and sister, mother and son, man and woman first cousins, etc. are often legally restricted from marrying, even though when both are adults they may meet the first part of "a man and a woman" in the definition of marriage. Maybe the jury is still out definitively on this matter. Maybe the definition of marriage needs to be supplemented with "a non-blood related man and woman" reflecting new relevant knowledge to be validly accurate. Regardless, procreation may or may not be a
requirement for marriage, but
a man and a woman as husband and wife is.
And so it is quite clear that two men, two women, two children, an adult and a child, an adult and an animal, and the like, are obvious extremes that far exceed rational inclusion in the definition of marriage.
Though topically irrelevant, removal of bias against women with regard to voting and such is an example of progress, as I doubt (if I recall correctly) the Constitution ever restricted women from voting or possessing any other realities of rights.
Whether erroneously inappropriate marriages have "done damage to the institution of marriage" is irrelevent. What matters is that errors were made and those errors need to be corrected or the definition of marriage re-defined.
As to marriage in our culture being about love and commitment, yes, a man and a woman as husband and wife would indeed do well to love and be committed to each other, but such is not in any way the
definitivelyminimal requirement for marriage, even though such may be implied in the phrase "as husband and wife".