The marriage equality movement needs to be for "all" marriages, and not just the ones we agree on.
Deciding what 'marriages' are allowed is a problem of definition. Before we can make it legal we have to agree on a definition of it. That must be done iteratively. We decide on one inclusion or exclusion at a time, simply because there are too many possible definitions to be sure we are fully inclusive, and because it is possible to find contradictory definitions that have to be chosen between.
Do we allow Same Sex marriage?
What about plural marriage? If so, what type of plural marriage? Line marriage, Group marriage, split marriage, single head hierarchical?
Can two people that are already married marry each other, if so is their spouses married to each other as well? Do those spouses need to consent, either way?
Is there a limit to how many people can be married, or could a US citizen marry 1.2 million people from Mexico (who now get US citizenship)? Do we allow incestual marriage?
Does everyone in a marriage need to be alive (BTW this is not a current requirement)?
Does everyone in a marriage need to be real, or could I marry my imaginary friend?
Do they need to be able to consent, or could a person in a coma be married?
Can a person be forced to marry via Court order?
Can a guardianship or Power of Attorney consent for a person?
Can a person be married in absentia?
Do we need to meet in person, or could we get married over IRC and never meet (this is currently allowed)?
This is just some of the questions. I could write literally hundreds more, just on plural marriage, given time. These are not easy issues, and it makes no sense to say that we have to have them all figured out before we do any of them.
If you are going to advocate for that position I hope you are also advocating for revoking all marriage until we are ready to make a comprehensive marriage law.