One popular justifications for SSM–in fact the primary justification–is the claim that so long as "two" people love each other, that's all that matters.
There seem to be a fair number of professing Christians who say that as well.
A couple of quick counterexamples:
i) In the pop culture, there are many people who think girlfriends and boyfriends can "cheat" on each other. Now, I'm out of step with the pop culture in that regard. I think only married couples can cheat on either other.
But let's grant pop cultural social mores for the sake of argument. If love is all that matters, why should boyfriends and girlfriends be exclusive?
ii) Then there's the trophy wife. Say a young man marries a young woman right out of high school. He's very ambitious. His goal in life is to be rich. He finagles his way into Stanford or Yale for his undergraduate degree. Then he gets an MBA from Harvard, or maybe graduates from Harvard Law school.
He then joins a top law firm or Wall Street investment firm. His wife worked a job to help put him through college. Perhaps they have two or three kids.
Now, however, he's far more eligible than he was in high school. The marriage has lost its passion. He falls in love with a blond bombshell. Women like her were out of his league when he got hitched the first time.
So he trades up. He dumps his workhorse for a showhorse.
BTW, when I was a kid, we used to call women like that "homewreckers."
As long as love is the litmus test, he did nothing wrong. It all comes down to falling in love and falling out of love. You switch partners accordingly. She served her purpose.
Do people who support SSM think he wronged his first wife?