1. In our society both legal and civil, we prefer women who have babies over women who don't have babies, therefore it is natural and right to prefer the relationship that creates the babies over the ones that can't and don't. For example, we allow maternity leave to women (and men) of newborns. This is clearly a preference for women with babies. Our tax code gives extra deductions for families that spend money on children, rather than spending money on other things. The list of prefences in our society is long and it is proper. Logically, it follows that we should have preferences for the relationships that create these babies. Our ultimate goal is to create a stable situation for men and women to procreate and perpetuate society.
2. Men and women generally complement each other. Whether it's men protecting women, or women domesticating men. This dynamic creates a more healthy situation for both genders. Society has an interest in maintaining and strengthening this bond. This can be done through law, custom, taboo, or whatever has evolved. Accepting other relationships as equal to a male-female bond naturally weakens the exclusivity and specialness.