And there is no reason not to give civil unions the same legal protections without calling it marriage.
So your argument is that an animal that looks like a duck, walks like a duck and quacks like a duck is a duck, but a gay duck is a civil avian?
What does having an additional term in the language to define the difference between heterosexual and homosexual marriage add to anything? Pro-homo marriage people will still just call it marriage either way, and anti-homo marriage people will still find ways to claim that it's an abomination regardless of the name. Can we just keep it simple for the rest of us ordinary folk?
I think you're fighting a losing battle, to be honest. Once it was to keep gays from being recognised at all, being gay was a crime. Then it was against gays having relationships openly. Then it was against gays being legally recognised as couples, and then it was gay couples receiving similar protections to hetero married couples. You're nearing the end of the line and all you've got left to fight for is the purity of the name "marriage". I wonder what you'll fight for when that's gone?
Let's face it, in another 50 years all the people who grew up culturally homophobic from the early 20th century will have died out, and all that will be left are people who grew up during times when gays were accepted. People who grew up with gay friends, work with gay people, have gay people in their family. It won't be possible to find many people who aren't connected to the gay community in some way, and it's pretty hard for most people to argue seriously for something that is a giant "🤬 you" to people that they actually like.
Best of luck to you, but your closed minded, biblically literalist way of thinking is about 400 years out of date. If you felt like opening your mind a bit, you might find that there's all sorts of great people out there and that all this stuff about which chromosomes you and your partner have is really pretty irrelevant.
What actually matters is having a nice life, being kind to those around you and building a community of happy, friendly people who aren't afraid to be who they want to be.