I know this family personally. These two men have adopted this lovely girl. They love one another, and the fathers are raising their daughter in a fantastically positive environment. When these two men cannot marry, it hurts their family.
As to how these two men being married would hurt the institution of marriage or any one else's straight marriage is beyond me. That I know and appreciate their example of a loving family, it is my firm belief that not only does their relationship do no harm to any one else, their commitment amplifies the strength of marriage all around.
Does proposition H8 hurt their family? Absolutely.
The vote of the majority should not be allowed to stand as it presents a tyranny over the minority. Simply because you can vote your "values" doesn't make you right, or your position correct. It's time to repeal Proposition H8.