Such response would be light years ahead of the muddle currently in the heads of the legislators, intimidated by homo thugs. However, there is one aspect missing.
True, two or more people should be able to grant one another any rights that they are in possession of. For example, they are automatic inheritance rights, hospital visits, medical care decisions, etc.
It is reasonable that they ask legal equality between various spacific instances of such union. For example, there is no reason to allow a gay couple to mutually granted rights and disallow two widowed sisters the same.
They cannot however demand that the state grants them privileges that the state chooses to grant properly married couples. Examples of such privileges are citizenship, taxation, adoption rights. That is because the heterosexual nature of the marital union is of a special value to the state, since in the event of children being produced or adopted by a married couple, the state has an interest in the children to be raised in a harmonious family, thus relieving the pressure on schools, juvenal law enforcement, etc. The state has no similar interest in non-sexual or homosexual or polygamous unions in the same way.
The demand of non-discrimination is valid between non-marital unions. It is not valid if a non-marital union is compared in privileges to marriage.
The fallacy of that “granted privileges” argument is that it depends on the premise that the people are there to serve the interest of the state rather than vice versa.