I'm late in adding my voice to this. California's Proposition 8, and similar efforts when they crop up in other states, destroys families. Its supporters like to argue in the abstract, but it has
real effects on real people, and if you can't look the affected people in the eye and say "yes, I intend to attack you", maybe you ought to rethink your support.
I am married, religious, and heterosexual. I cannot see what recognizing other types of unions could possibly do to threaten my marriage. On the contrary, equal acknowledgement of all unions helps protect the institution; it makes it more likely that the folks in marriages actually want to be in them, rather than settling just to get legal protection (for, say, your kids).
What threatens marriage? Taking it lightly and not working with one's partner(s) to strengthen the family. The high rates of divorce and abuse demonstrate that we heterosexuals don't have a great track record on this. Why should I believe that my gay friends will do worse? I expect they'll do better, because when you're a minority, it takes a certain degree of commitment to your marriage to be willing to put yourself out there in the first place. I suspect there is a far, far lower proportion of casual marriages in the gay community than there is in mine.
You know what consittutional amendment I'd like to see? The abolition of marriage as a legal entity. The avenue of legal partnership -- for the sake of inheritance, custody, power of attorney, taxes, finances, etc -- should be available to any group of people who voluntarily and compently choose to enter into such an arrangement. The state should simply register them, as it does for business partnerships. Beyond that, it's not a state concern. This is not marriage; this is a civil union.
Marriage, on the other hand, is a religous matter. Different religions have different rules for what they will and won't accept. That's fine; all communities have rules that apply within that community. It is equally valid for Roman Catholics to say "no divorcees need apply", for Jews to say "no intermarriages here", and for Pastafarians to say "marriages must be trios of any two adults and a pasta product". Your community, your rules, and your own enforcement problem. Please leave the rest of us out of it.
If there is anyone out there who is at this late hour still able to turn dollars into efforts to defeat this proposition, please let me know. (The link I've seen expired before I saw it.)