Marriage, as a civil institution, is given favored status under the law for the specific purpose of stabilizing society and raising children.
These disordered lifestyles do nothing to stabilize society or raise children up as future productive citizens. Therefore they should not be given preferential tax status, which is the primary advantage of “marriage”. Property rights, inheritance, etc, can be accomplished without “marriage”.
If society is going to confer a status on individuals that gives them a tax and other benefit advantage, society expects a return. This group gives no return. They do not need an incentive to procreate and they do not need tax preferred status to live in the same household.
Edited to add: “equal rights” is a complete red herring. Every adult in this country has the same legal right to marry. The government restricts who can validly join in a “marriage”. The government has that right. They can restrict Billl, Sue, and Ted from joining together in a “marriage” and the tax and other benefits these three receive as a result of attempting to claim that they are all three “married” to each other.