I could grant my partner (my wife, in my case) any kind of special legal status I want. For instance, she might designated me her health-care proxy, and I could do the same for her.
Similarly, two men could designate each other with all sorts of special relationships under the law through mutual contract, and do so all the time. If this constitutes marriage, we've had gay marriage for the last twenty years.
Indeed, by completing the appropriate legal paperwork (as though your wife were any stranger that you happened to want to endow with such things.)
But if you do not want government to define marriage, then there can be no automatic vesting of rights (e.g. automatic medical power of attorney) to someone just because you are married to them.
Sounds like you are okay with that. Not everyone is.