You're actually getting a little mixed up with the comparison here.
It's true that one doesn't choose to be a woman or to be black. By the same token, one doesn't choose to be gay. As Elder Oaks put it, "Some kinds of feelings seem to be inborn. Others are traceable to mortal experiences. Still other feelings seem to be acquired from a complex interaction of 'nature and nurture.' All of us have some feelings we did not choose" (Ensign, Oct. 1995, "Same-Gender Attraction").
The stance of the Church is that homosexual attraction is not a choice. Homosexual behavior is, but homosexuality itself (like heterosexuality) is not.
Now, you're right that living a gay lifestyle is a choice. Homosexual behavior is chosen. And you're right that Prop 8 was about homosexual behavior rather than homosexual attraction.
But laws on blacks' rights and women's rights weren't about being black or being a woman. There has never been a law in America against being black or female. The laws have been against certain behaviors among blacks and females. You don't choose to be black, but you do choose to vote, or to use a certain drinking fountain, or to ride in a certain train car, or to attend a certain school. You don't choose to be a woman, but you do choose to vote, or to attempt to acquire property, or to enter a certain profession, or to get a divorce.
Movements for racial and sexual equality have never been about how the law views the issues that aren't a matter of choice. They've focussed on the issues that are a matter of choice. The question has always been this: should people in one unchosen category be free to choose behaviors that we allow of people in a corresponding unchosen category?
Since we know from our Church leaders that homosexuality is an unchosen category, it's perfectly legitimate to compare issues like Prop 8 to movements for racial or sexual equality.
Great post!:clap