The US constitution guarantees equal rights for every individual, regardless of race, creed, gender, or any other individuating factor. There is absolutely no reason to deny any one person, or a group of people, a civil right that is offered to every other person.
Banning a person (or persons) from any governmental civil service, based on their personal trait, belief, or lifestyle, is inherently unconstitutional. America was formed with the freedom of every individual a right, no matter another person's opinion.
We cannot allow institutionalized inequality in this country ever.
This is a battle that many groups in America have fought and indeed are still fighting.
Proposition 8 is unconstitutional, and voting in its favor is un-American. It's that simple.