This is from the Religious Tolerance Website: http://www.religioustolerance.org/mar_bene.htm
On the order of 1,400 legal rights are conferred upon married couples in the U.S. Typically these are composed of about 400 state benefits and over 1,000 federal benefits. Among them are the rights to:
joint parenting;
joint adoption;
joint foster care, custody, and visitation (including non-biological parents);
status as next-of-kin for hospital visits and medical decisions where one partner is too ill to be competent;
joint insurance policies for home, auto and health;
dissolution and divorce protections such as community property and child support;
immigration and residency for partners from other countries;
inheritance automatically in the absence of a will;
joint leases with automatic renewal rights in the event one partner dies or leaves the house or apartment;
inheritance of jointly-owned real and personal property through the right of survivorship (which avoids the time and expense and taxes in probate);
benefits such as annuities, pension plans, Social Security, and Medicare;
spousal exemptions to property tax increases upon the death of one partner who is a co-owner of the home;
veterans' discounts on medical care, education, and home loans; joint filing of tax returns;
joint filing of customs claims when traveling;
wrongful death benefits for a surviving partner and children;
bereavement or sick leave to care for a partner or child;
decision-making power with respect to whether a deceased partner will be cremated or not and where to bury him or her;
crime victims' recovery benefits;
loss of consortium tort benefits;
domestic violence protection orders;
judicial protections and evidentiary immunity;
and more....
Most of these legal and economic benefits cannot be privately arranged or contracted for. For example, absent a legal (or civil) marriage, there is no guaranteed joint responsibility to the partner and to third parties (including children) in such areas as child support, debts to creditors, taxes, etc. In addition, private employers and institutions often give other economic privileges and other benefits (special rates or memberships) only to married couples. And, of course, when people cannot marry, they are denied all the emotional and social benefits and responsibilities of marriage as well.