I started writing a reply to Ihlin's Mom on another thread, but it got long, and it was way back in the diaries ....
Rather than the civil rights view, which I still hold personally, take the civil liberties view, which I proudly espouse.
Democratic governments should mainly serve to PREVENT things. We decide what they should PREVENT because these things harm OUR rights. Stealing from merchants. Tax cheating. Murder.
Abortion is a sticky wicket, because religious people feel it is murder. I still believe women should have that right, but that's another issue.
Civil Libertarian Progressives, like all of the guys in the Democratic field today (not all of whom are CLPs), support Civil Unions. However, gay marriage would also be an option for a free government. Why?
Because it doesn't harm our rights.
We allow a lot of sins in this country. I'm sure your mother doesn't agree with all of them, or like them. I don't, especially the ones that are medically destructive. Some of them even infringe on my rights as a taxpayer ... and I'd like to curb them or aid people to recover, but not ban them or turn their purveyors into second class citizens. AND I'm a Jew, so I have a lot more things than 10 commandments and 7 deadly sins. I have 613 laws or so. Free governments should enable rights. Gay marriage doesn't take away ANY of her rights.
In addition, civil unions or gay marriage (heck, I'd call the government end of MY marriage a civil union if I had the same rights) help keep marriage alive as an institution. It teaches children that governments only secure certain rights for people who sign contracts, so that those rights (especially economic ones) are not abused by quickie couples ... and so both parties in the partnership have legal protections should they dissolve their union.
Your mother, being reasonable, understands that children will see homosexual couples on the street. She has one of two options. She can lecture that child that she believes homosexuality to be wrong, but also say that those people are married. Or she can say that they aren't married to boot. That child will either be gay or not, but if he/she realizes that people can legally live together without marriage, it destroys marriage. Even if you believe that the child can CHOOSE not to be gay, he'll still see gay people. So, either you want to put burqas on gay people, or you'd better make sure that you explain that couples sign a piece of paper. Of course, you're lying, unless you permit the government to permit homosexuals to get a piece of paper to sign.
What you do in your church is your business. I think we have to respect these people's beliefs, because their beliefs don't harm rights either (unless they are old-school Aztecs or something). And comparing her to Pat Robertson is divisive and unproductive. Don't do that.
At the same time, I urge all good civil lib progs to use these arguments:
A: The French BAN religious symbols. We aren't doing that. We respect religion. You can express your religion peacefully. We're not French.
B: Forget Pat Robertson (so 80s!). Muslim extremists enforce religious law. Sins are legally punished. We're not Saudi Arabia or Sudan, or some other Sharia Law Islamo-Totalitarian State. We're better than that.
I was trained by the best. A little wizened creature from Vermont riding in on the back of the pace car at the Daytona 500 with a Confederate Flag DECAL on the back.