Fundamentally, there is one aspect of marriage which has been essentially universal throughout history among nearly all societies which continued to thrive after the death of their original founding members: every woman is recognized as either being unmarried, or having exactly one husband, and in the latter situation shall be forbidden from having sexual relations with anyone but the husband without the husband's consent (which he may very well never give). Every marriage must thus involve exactly one man and at least one woman. A union of two men would involve two men and zero women; a union of two women would be slightly less abnormal, and might conform to some societies' norms if the two women brought in a husband.
Well put.