Tax Benefits
Filing joint income tax returns with the IRS and state taxing authorities.
Creating a "family partnership" under federal tax laws, which allows you to divide business income among family members.
Estate Planning Benefits
Inheriting a share of your spouse's estate.
Receiving an exemption from both estate taxes and gift taxes for all property you give or leave to your spouse.
Creating life estate trusts that are restricted to married couples, including QTIP trusts, QDOT trusts, and marital deduction trusts.
Obtaining priority if your spouse needs a conservator--that is, someone to make financial or medical decisions on your spouse's behalf.
Government Benefits
Receiving Social Security, Medicare, and disability benefits for spouses.
Receiving veterans' and military benefits for spouses, such as those for education, medical care, or special loans.
Receiving public assistance benefits.
Employment Benefits
Obtaining insurance benefits through a spouse's employer.
Taking family leave to care for your spouse during an illness.
Receiving wages, workers' compensation, and retirement plan benefits for a deceased spouse.
Taking bereavement leave if your spouse or one of your spouse's close relatives dies.
Medical Benefits
Visiting your spouse in a hospital intensive care unit or during restricted visiting hours in other parts of a medical facility.
Making medical decisions if your spouse becomes incapacitated and unable to express wishes for treatment.
Death Benefits
Consenting to after-death examinations and procedures.
Making burial or other final arrangements.
Family Benefits
Filing for stepparent or joint adoption.
Applying for joint foster care rights.
Receiving a share of marital property if you divorce.
Receiving spousal or child support, child custody, and visitation if you divorce.
Housing Benefits
Living in neighborhoods zoned for "families only."
Automatically renewing leases signed by your spouse.
Consumer Benefits
Receiving family rates for health, homeowners', auto, and other types of insurance.
Receiving tuition discounts and permission to use school facilities.
Other consumer discounts and incentives offered only to married couples or families.
Other Legal Benefits and Protections
Suing a third person for wrongful death of your spouse and loss of consortium (loss of intimacy).
Suing a third person for offenses that interfere with the success of your marriage, such as alienation of affection and criminal conversation (these laws are available in only a few states).
Claiming the marital communications privilege, which means a court can't force you to disclose the contents of confidential communications made between you and your spouse during your marriage.
Receiving crime victims' recovery benefits if your spouse is the victim of a crime.
Obtaining immigration and residency benefits for noncitizen spouse.
Visiting rights in jails and other places where visitors are restricted to immediate family.
Same-Sex Marriage, Civil Unions, and Domestic Partnerships
On June 26, 2015, the U.S. Supreme Court decided the historic Obergefell case and ruled that same-sex marriage bans are unconstitutional, and same-sex couples can legally marry anywhere in the United States. If you are in a same-sex marriage, your union will be legally recognized everywhere in the United States, and you are entitled to all of the same state and federal benefits as opposite-sex married couples.
However, these rules do not apply to unmarried couples that have established either a domestic partnership or civil union. If you are in either of these two marriage-alternative unions, none of the benefits of marriage under federal law will apply to you, because the federal government does not recognize these same-sex relationships. For example, you may not file joint federal income tax returns with your partner, even if your state allows you to file your state tax returns jointly. And other federal benefits, such as Social Security death benefits and COBRA continuation insurance coverage, may not apply.
To learn more about the rights and benefits available to same-sex couples, consult a lawyer with expertise in this area and see Making It Legal:A Guide to Same-Sex Marriage, Domestic Partnerships & Civil Unions, by Frederick Hertz with Emily Doskow (Nolo).
It's good that this works for some people, but I doubt most people would enjoy that type of arrangement. But then again, I'm old school. What seems odd now, might become common in the future.