Well, just ask yourself, would any straight people ask for a civil union instead of a marriage?
Non-discrimination, equal protection, and equal rights are a desirable goal and can be achieved by a civil union. But you can not legislate morality. And as long as whether or not homosexuality is all right or is wrong rides fundamentally upon the issue of one's religious beliefs (or lack thereof), then forcing me to accept it as morally right by legislating state-sanctioned approval is discrimination against me and my beliefs. Furthermore, the Constitution says the government will make no laws regarding the establishment of religion. If marriage is a religious/beliefs issue, then making a law legalizing homosexual marriage is simply unconstitutional, because it's the state getting involved with setting religious beliefs.