Right—same with race, creed, and gender. That's the theory, at least. How fearful do you expect, say, the Super Bowl–winning Seattle Seahawks are of a lawsuit alleging racial discrimination on the grounds that its defensive squad is biased against non-blacks?
http://fantasyfootballwarehouse.com/wp-content/uploads/2013/...
In practice, the treatment of "underrepresented" minorities vs. "overrepresented" groups is almost completely asymmetric.
Prop 8 is an attempt to remove rights from a group of people.
You can't remove a right that doesn't exist. Even if you're generally sympathetic to gay rights (as I am), the idea that the framers of the California state constitution intended to protect the right of two men (or two women) to marry each other is risible.
The proper analogy would be a hypothetical Prop 88 which seeks to ban straight marriage while allowing gay marriage.
You're assuming that gay marriage and straight marriage are equally valid relationships. This may be true, but the whole point of Prop. 8 was an explicit rejection of this premise. By positing a symmetry between gay and straight marriage, you're simply begging the question.