To my knowledge, there are no laws that prevent two adults (or more for that matter) to enter a relationship, live together, do whatever together, share everything, raise any resulting children etc.
There may be laws that don't recognize the special relationships even friends might have, that, for instance, allows only "immediate family" to visit someone in a hospital, or to put someone else on their health insurance policy.
These are all issues that can be addressed without redefining the word "marriage" for a culture that rejects the proposed new definition. These kinds of solutions have been proposed, by creating "legal unions" or "civil unions" as contracts and knit social units recognizable by the state. But that is, again and again, not what the activists want. They've rejected that route. They want the word. It may sound trivial, but it is not. The word, once it becomes a legal definition, will be used to bludgeon those who don't recognize the unions as "marriage" and as a result refuse to treat it as such.
Rand Paul espouses a solution I have talked about on many occasions. Take the word "Marriage" out of the tax code - and just enforce contract law, of which marriage is one of many.
The problem is, is that the proposed solution, the only one anybody but a few like Rand Paul are talking about, is to literally change the definition of the word for a a culture that roundly rejects that redefinition. It is the ultimate in cultural insensitivity. But if it were just that, I would have much less of a problem with it.
Now I'm not saying people cannot call their own relationships anything they want to call them, but it is quite another thing to force someone else to call it something they don't recognize it as. This is a special interest group using government power to stomp on existing culture.
If you don't think this is a problem, read Ed Morrisey's article -- I'll excerpt the part I'm talking about here. File it under "Oh, THAT'LL Never Happen".
"Tolerance, it seems, works only in one direction — and that brings us to the religious argument, but not in the manner one might think.
While as a practicing Catholic my definition of marriage involves its sacramental character, I understand that others may not share my faith and perspective on its meaning or value. That, however, will not work both ways, as recent examples have made plain. For example, a baker in Oregon faces potential criminal charges for refusing to provide a wedding cake for a same-sex couple because of his religious beliefs. What happens when churches refuse to perform such ceremonies for the same reason?
Most people scoff at this question, but religions have partnered with the state on marriages in a way that bakers have not. Priests, ministers, rabbis, and imams act in place of the state when officiating at wedding ceremonies, and states that legalize same-sex marriage are eventually going to be forced by lawsuits to address that partnership, probably sooner rather than later. In similar partnerships, that has resulted in pushing churches out of business."Got that? If you don't agree with the government definition for religious reasons, tough luck, buddy. Bake 'em a cake, place kids with them even if you believe it is wrong according to your religion -- or go to jail. The First Amendment means nothing.
And apparently neither do legitimate democratic public referendums.