system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If there is a separation of church and state, why are there laws that prevent gay marriage?

just wondering... thanks!
gpt
Well bud, since our governments have laws that allow special rights and privileges to married couples they need to have a legal definition of marriage. Mommy and Daddy couldn't say "We're married" and not go through the legal process of becoming husband and wife and expect those same rights to be given to us. We have to go through the process.

This is why the preachers have to get special permission to perform weddings and they say things like "by the power invested in me by the state of ____" when they pronounce a couple married.

You can claim to be married all you want, but if it isn't done the way the state says it has to be done you get no legal marriage rights. In the same way the state has the power to say who can and can not be married. 

Since people vote for the representatives of the state they can try and vote people in that will support their own personal view of marriage and on this issue the loudest most outspoken groups are the ones who are for gay marriage and those who are against it. Typically those against it are aligned with some religious group and this is why you see the church involved in the debate, but in actuality the church is not the authority on who is married or not anymore, which is why you can go and get married in a court house by a judge, or by a non-minister who's been certified as an officiant for weddings by the state.

tl;dr: Modern Marriage isn't really a function of the church anymore. It's really a Government issue.