Okay, this "one thing in the whole wide world" effects
-Adoption
Its easier to adopt children when you are married as opposed to single.
-Parental rights
If the child/children are bilogically speaking, one of the two partners, then the grandparents have more rights than the other spouse in case of their death.
-Health care
In states where their marrige isn't legal, the other spouse cannot use their health care. In the case of my mother and father, they were able to pool their healthcare together for his liver transplant. A homosexual couple cannot always do the same.
-Survivors annuity
When my dad died, since he was a government employee, my mother recieves a certain amount each month. A homosexual couple doesn't always recieve this.
These are just off the top of my head. Not to mention any other rights spouses usually get, like not being barred from a funeral just because the other spouses parents disagree with their lifestyle