Well there are a lot of things that fall under "liberty and the pursuit of happiness" that are limited by the law for good reasons. Even anti-drug laws, which many would say citizens should have the right to harm themselves if they want to, have an argument for there being a state interest in promoting the health and mental clarity of the population. Not everyone will agree that the state's interest is sufficent to justify limiting the rights, but the argument is there, and the courts (for the time being anyway) support it. You don't have the right to steal from your neighbor even though his x-box might make you happy. You don't have the right (bringing it back to family law) to refuse to support your children just because you would rather spend the money elsewhere. Every law in existence is there to limit your liberties, under the premise that the law creates a civilized society which everyone benefits from. Living in the society, you agree to having some freedoms restricted in exchange for the protection the law can offer you. I don't have a problem with the existence of laws, I'm not one of those crazy militia types like the whackos on CSI last week
However the other premise that this country was founded on is that all human beings were created EQUAL. And there is absolutely no compelling reason that SOME people should be allowed to marry the person of their choosing, the person who they love and wish to build a life/family with, but others can't. I can see a state interest in not allowing poligamy; but like I said, no material difference between 2-person marriages between same sex or opposite sex couple. If we are all equal, then it's clear marriage is one of those rights that needs to be available to everyone.