the institution of marriage pre-dates recorded history and in all that time, it has always been between men and women. There is nothing unconstitutional about wanting to maintain that 20,000 year precedent.
And it shouldn’t be any of the gov’t business.
In the United States, until the mid-19th century, common-law marriages were recognized as valid, but thereafter some states began to invalidate common-law marriages. Common-law marriages, if recognized, are valid, notwithstanding the absence of a marriage license. The requirement for a marriage license was used as a mechanism to prohibit whites from marrying blacks, mulattos, Japanese, Chinese, Native Americans, Mongolians, Malays or Filipinos. By the 1920s, 38 states used the mechanism.