The rules are the same for all — anybody is entitled to marrying one person of the opposite gender. Some people aren't able to use that right, but that's not a reason to redefine the meaning of marriage.
Not long ago, everybody had the same right to marry a person of the same race. Some people didn't want to make use of that right, and it caused a ruckus, and eventually we granted them some crazy new rights. Was Loving v. Virginia decided incorrectly? Was the system fair and equitable as it was before Loving, and were the agitators agitating over nothing?
Based on what I've seen out of some justices (Scalia, I'm looking at you), their arguments make me think that they think Loving was incorrectly decided. Because its not popular, they say that it was correctly decided, but everything else they say and do make me think otherwise. Every decision and dissent Scalia has written on sodomy and race makes me think that he'd go the other way on Loving.