The "Secular" Argument Against Gay Marriage
What is the purpose of marriage as a state-recognized institution? Anybody can put on fancy clothes, rent a venue, invite friends and family, and make promises to each other. Why have "marriage" as a legal thing?
Most people see marriage today as an expression of love, or as the formalization of a relationship. If that's all it is, then there is no reason for the government to be involved, and there is no "secular" reason to not recognize gay marriage, polygamy, human-animal marriage, and so on.
However, the actual purpose of marriage throughout human history, across millennia, across various cultures and religions, boiled down to two things:
Man and woman partner up, woman gets pregnant. Now she's in a weakened state, may or may not be able to engage in sexual activity, and her partner may see her as less attractive (yes, I know some men find their wives more attractive, just follow me here). She needs to know that he is going to stay with her, protect her, and provide for her during her pregnancy and once she has a newborn.
The child also needs the assurance that his or her father will stay around and provide for the family.
Marriage, up until the 20th century, was about providing assurances for women and children that they would be provided for. It ensured that inheritance went to them if the father died, and that if he abandoned them they had legal rights to alimony/child support.
It was in the 20th century that we started divorcing (pun intended) marriage from its actual purpose. We started saying it was just about feelings, which is why no fault divorce came in. If marriage is about a feeling, why stay in the marriage if the feeling is gone?
Marriage was ALWAYS an institution designed to protect the weak and vulnerable. Which is why the concept of gay marriage wouldn't compute for anyone prior to about the 20th century. Not that gay relationships - secret or otherwise - didn't exist. No one would see a reason to use the institution of marriage for those relationships, because A) no children would be produced and B) there is either no woman in need of protection/provision, or no man to protect/provide.
The whole point of marriage is to put guardrails around male-female relationships to balance out the power difference between the sexes. Men are physically stronger, men face less risk from sexual activity (they don't get pregnant and often are less likely to have severe symptoms from STIs), and men typically are the higher income earners, even today.
It's simply not relevant for same-sex relationships.
Now, when it comes to things like taxes, hospital visitation, and inheritance, those things can be handled really easily under our current system without involving marriage. A living will can be used to give someone medical power of attorney and visitation rights. That person can be a friend, family member, roommate, whoever you want. A regular will is something that anyone with any kind of assets (house, savings, etc) should have, whether you're in a gay relationship or not.
But, back to the marriage issue. It's not just about gay marriage - no-fault divorce is what opened the door by undermining the purposed of what marriage is. It allowed men to more easily abandon their wives, and it also allowed women to fraudulently take their husband's assets and earnings and run off for their Eat Pray Love self-discovery adventure.
I hope that Obergefell is overturned, but I also hope that we can re-establish the actual purpose and function of marriage - ensuring women and children are protected and provided for by their husbands/fathers.