The problem with your argument is that people are *not* asking the government to create or mandate a "marriage law" as you put it. Nobody who is actually working in the courts and legislative system is asking for any particular right. Nobody is asking for that, in any appreciable numbers, or with any appreciable influence.
What IS happening, is that people are asking the government to clarify that there is no right to marriage at all.
The so-called "right" to marry who you want is what is known as an "unenumerated right," meaning it's a "right" that you have by default, with no pre-existing restrictions, conditions or provisions. It's like the air around your head: it's yours to use however you see fit, so long as it doesn't impinge upon someone else's free use.
What has happened is that individual states have illegally declared marriage to be an enumerated right that is the exclusive domain of a particular majority of society: i.e. heterosexuals.
That, along with the Defense of Marriage Act, is in direct contradiction to the Constitution.
So what the lawyers, activists and people with their hands in the issue are *really* asking for, is for the federal government to step in and say, "Marriage is not an enumerated right. Constitution wins, you lose, obey the law of the land. Allow consenting adults to marry whatever other consenting adults they wish."
(With the appropriate, already established and legitimate conditions regarding age, consent, genetics and being an actual human.)