(I promise that this is about gay marriage but I will take a little bit to get there.)
I think that the most sensible argument about for hetero only marriage laws is the tradition behind it.
The most "traditional" marriage are those that both sets of my grandparents had. Both of my grandmothers stayed at home and raised the kids and my grandfathers worked and brought home the money. My grandmothers both volunteered with various clubs or activities but their main duty was tending to their homes and their children. Their marriages and lifestyles were similar to their grandparents (my great-great grandparents) and likely many generations back. This is how things have always been done.
Let's fast forward to the women's liberation movement in the 60s and 70s. It flipped a ton of traditions on its head. Were now going to school in other things than just liberal arts and teaching. Women could enter the workforce and become a dual-income family or not even need to get married and be able to support their self.
My parent's marriage looks a lot different than my grandparents. My mom even made more money than my dad for a number of years because she had a masters in statistics and worked as a statistical programmer. They split household chores, more of it still fell on my mom but at least they were somewhat split.
So in one generation the meaning of marriage completely changed what had been done for centuries... even millennia. Slow baby steps and one final larger step lead to women not needing a husband to support their self.
The one thing that looks the same from my grandparents marriage and my parents marriage is that it's between two loving opposite-sex partners. I would still consider my dad to be the head of household and lead the family unit but my mom was the one that kept everything going would freely speak her mind to him - something my grandmothers didn't do as much of.
In 100 years, we have "evolved" as a society. Women hold equal power as men in most areas. My grandparents have witnessed all of this in their lifetimes. The better rights for women, grandmothers could get behind because some of them wished it for their lives and are happy that their children get to live it. But marriage, it's always been between a man and a woman to start their "family unit".
In their mind that's what marriage is... it's a tradition that has always been a tradition between a man and woman. The gay rights movement isn't one that is relatable existing hetero marriage couples so why should they support it? It's never been done that way. Marriage to them is a social construct to them and it has always been.
I can see at least see why they get their panties in a bunch about the social construct of marriage.
Of course the obvious argument to this is that it's also a legal construct - and it always has been that, too. Not allowing gay people to get married is excluding them from legal rights and that isn't fair.
We have come a long way in three generations.