If men and women are equal (m=w), then a man and a woman is equal to two men or two women (m+w = 2m = 2w). To afford opposite-sex couples special status is therefore based in gender inequality.
Until relatively recently, the law treated husbands and wives differently. The case could be made that restricting marriage to opposite-sex couples is a vestige of that history. You could also point to Ancient Greece, where they didn’t exactly have a problem with homosexuality, but still restricted marriage to a man and a woman, on the basis of treating men and women differently.
This is an excellent point. It may also partly be a vestige of the history of marriage (and sex) merely as a means of producing offspring, a view many or most would consider outdated or worse today.