Every time this argument rears its ugly head, I like to remind everyone of one simple thing.
If your church is really against gay marriage, it doesn't have to perform those ceremonies.
No, really, your church can marry whoever it wants.
The reason it's important to permit gay marriage is because this will give equal rights to gay couples. Countless laws are already in place that specify 'marriage' regarding couples' rights, not civil unions. If you want to give civil unions the same rights as marriages, that is going to require a shit ton of unnecessary work across the country, rewriting these laws and getting them passed. Sorry, but that's ridiculous. Allowing gays to get married is no different than giving them civil unions + the extra rights that all couples deserve.
With that in mind, anyone who seriously does not support gay marriage is either grossly misinformed or a bigot. Denying a group the same rights because you find that group icky is bigotry, pure and simple. Churches don't have to perform marriage ceremonies, but the government should recognize marriages between gay couples.
Allowing 'civil unions' is no different than the 'separate but equal' phrase used so often after the civil war. The problem is that the facilities are separate, but they're not equal. Worse yet, there's no reason for the separation between civil unions and marriage. It's completely unnecessary. Allow gays to be married, let's be done with it already.
This is why we have courts, to prevent the tyranny of the majority. This is how black segregation was struck down in most of the south, and this is how gay marriage will become a national norm as well. Denying two men the right to marry is no different than denying two blacks the right to marry (they can have a civil union - sorry, no). You're denying someone a set of rights based on arbitrary reasons.