Based on what you seem to think marriage has evolved to is a father and son “married” is a “mother and daughter” married?
1: It is a fallacy to assume that because I am able to explain the positions of others, that I agree with their position. For the record, I completely disagree for a number of reasons, including those you state.
2: Nonetheless, the majority of SCOTUS disagrees, and that makes it law in the USA (right or wrong).
3: After reading the opinion (in part), I can not see any justification in the opinion to not include in the 14th Amendment rights the liberty of any consenting adults who wish to have a loving (even if not one that would involve the act of procreation) relationship. Denying the formalization of such a relationship via marriage would infringe on their “individual autonomy”, and the state would need to pass strict scrutiny (which IMHO it would not pass) to deny 14th Amendment rights of “equal dignity” before the law.