No, it couldn't. My argument is that a law is discriminatory if it prohibits certain actions to one group, but not to another.
Laws against interracial marriage prohibit the marrying of a white woman to black men while allowing it to white men.
Laws against homosexual marriage prohibit the marrying of a man to men, while allowing it to women.
Laws against polygamy prohibit the marrying of more than one person to everybody*.
What you are trying to define the certain action as whatever someone happens to want to do, as in "Laws against polygamy prohibit polygamists from doing what they want to do while allowing monogamists to do what they want to do. This discriminates against polygamists." The problem is that what they want to do is not a constant. You are not denying and allowing the same thing.
To accept this as discriminatory, you would have to accept all prohibitive laws as discriminatory, at which point the word loses any useful meaning. "Laws against stabbing puppies prohibit puppy-stabbers from doing what they want to do while allowing puppy-petters to do what they want to do. This discriminates against puppy-stabbers."
*I suppose you could say that polygamy laws do disciminate against the already married, as they are in fact prohibited from marrying, whereas single persons are not, but by definition married persons are already exercising their right.