Marriage, legal marriage, is a legal status. They want to deny that.
I doubt most of these people want gay people to have the same rights as other people. A lot of these people use religion to make their prejudices acceptable.
Look at the bakery situation. They didn't want to make a cake for gay people. And they got a LOT of support, especially in the religious community. Same with that woman who refused to give a marriage license to gay people.
You can say that they're not against gay people all you like, but it's not true.