What benefits attend to the State recognizing and defining marriage, as well as other relationships that are not marriage? Ideally the State should be an agent to promote prosperity and good order. The State defines a multitude of relationships and commitments. It honors faithfulness, truth, and other virtues the left would like to define or legislate out of existence. There is an inherent interest for the State to distinguish between virtue and vice, using its authority to punish evildoers and reward those who do good - just as it is in a good, solid home with God-fearing children and parents.
BTW, no matter how much “love and commitment” may be present before, during, or after the act, there is no virtue in depositing one’s own DNA into an excretory orifice. There is most certainly an ill effect and mendacity against the Creator and nature in defining “marriage” as equal to such a practice. The left cannot handle truths of this nature, as it elevates an imagined “inclusiveness” over the exclusiveness of virtue.