There are thousands of rights that mariage brings. The big rights that marriage provides are for parenting, medical protection, retirement and inheritance.
Married people can easily adopt children and merge families with equal legal custody. If one parent dies, the surviving spouse and children are protected by government assistance and private insurance.
In the event of medical crisis, the well spouse can legally act as the advocate and decision maker for the incapacitated spouse.
Most medical insurance plans will cover a spouse, but will not cover a boyfriend. Other types of insurance, like auto, may have similar restrictions.
During retirement, spouses can legally share in joint benefits (private pensions, government old age support), even after one of them dies. This right protects people in their old age. Gays who do not have this benefit suffer far worse poverty when they get old.
When one spouse dies, if a will is not available, property and wealth passes to the survivor. If you are not married and your partner dies, his wealth passes not to you but to his family.
But most important, where I live, my common-law spouse and I can buy a family fishing license, which is a lot cheaper than buying individual fishing licenses.