Marriage for LGBTQ persons offers many legal benefits, responsbilities, and protections:

• Marriage offers 1,138 Federal benefits and responsibilities, not including hundreds more offered by every state.
• Financial issues are complex and challenging, no matter the couple. And when home ownership, kids and other assets are a part of the equation, planning for the present and especially the future is even more critical for greater security.
• A child who grows up with married parents benefits from the fact that his or her parents' relationship is recognized by law and receives legal protections.  The mark of a strong family and healthy children is having parents who are nurturing, caring, and loving. Parents should be judged on their ability to parent, not by their age, race, religion, gender, disability, sexual orientation or gender identity.
• Studies show that people who are married tend to live longer and lead healthier lives.

From Why Marriage?  Marriage Equality USA