I'll give you a real life scenario. Trust me this is real life, not hypothetical.
A man and woman have been together for about 17 years and engaged for 10.
In the last year the woman has undergone several medical treatments and has now been diagnosed with a form of cancer. More treatments are coming this year. Her medical flex account is used up and now big dollars are going to be required for her care.
The man knew he needed to get her on his medical insurance and the easiest way to do it was to get married. He lamented briefly that it wasn't the way he ever wanted to do it but at this point there was no other choice. They will get married soon to remedy the insurance situation.
Who wouldn't do this if they had the choice?
If I took this same scenario and made it two men, would it make a difference to you? I'm not telling which scenario is true for the sake of discussion.
I've often proposed that government should not be in the business of granting the right to marry. Personally I'd turn ALL unions into civil unions. Once more I'd open the floodgates for all types of civil unions.
My Grandmother and her sister lived together for several decades until the end of their lives. I honestly believe my grandmother could have benefited from a civil union that would've allowed their unique relationship to have the status of a family household with all of its benefits. I'm also demonstrating in Libertarian terms how reducing the role of government in this case can ensure legal equality.
I honestly don't know what would protect the sanctity of marriage but I can't bring myself to believe that any of these scenarios would hurt it.