Marriage gives your spouse not only a legal right to inherit without property tax, but a legal right to inherit full stop, and without going through probate. It's a legal right that can't be interfered with by meddling relatives who are trying to overturn the will. Likewise, married people have the right to make legal and medical decisions for their spouses if the spouse is incapactiated, and the right to make decisions about burial. Also, married people can draw on the social security and retirement benefits of their spouse, which is important if one partner is out of the workforce to care for children/elderly family members. They can far more easily set up parenting and visitation agreements for children who might not be biologically or even legally the child of both spouses (ie. foster kids), a whole host of legal protections including the right to recover in a wrongful death suit or a tort action if someone kills or permanetly injures your spouse, crime victim's recovery benefits, the privilege of not being forced to testify against your spouse in a court of law, customs and immigration benefits... need I go on?

All in all, there are 1049 Federal legal benefits that accrue to married couples which do not accrue to cohabitating unmarried couples and can't be created through contracts. There are also state benefits depending on your state of residence.

So, to answer the unasked question: YES, same-sex marriage is a BIG DEAL.