I used to support civil unions for same sex couples until I thought more about what that implied. It's all well an dandy to allow people to contract to give each other hospital visitation and inheritance rights, and you can/should be able to do that between any two people regardless of relation.
But when you get into things like tax benefits and ability to refuse to testify in court against your spouse, you are actually asking the state to subsidize and promote that behavior. Placing that relationship above even justice in criminal cases. States can and should have the right to decide for themselves whether they think sodomy is worth subsidizing. (The benefits of subsidizing heterosexual relationships is clear hopefully)