Apart from it being a Religious term (in the Bible, it mentions that marriage is between a husband and wife, being man and woman). That's part of the base scripture.
The bible uses a lot of words. That doesn't make them all religious terms. You can have non-religous marriage.
A possibly less confrontational way round this is to just start a religion that does't have the figures that have said things in conflict with what you want to happen, and go with that, and its marriage systems and so on.
Do you realize that many gay people are religous, go to gay-friendly churches, and have commitment ceremonies in those churches?