At what point does a good faith disagreement become taboo?
Edited: A closer analogy would be interracial marriage. Laws prohibiting interracial marriage weren't struct down until 1967. Could a respectful person advocate banning interracial marriages ?
It should be never.
Let's take your example. For sake of argument I'm going to make an argument against interracial marriage.
The US has a great diversity of cultural identities which are largely tied to race. For example, African Americans make up 13.2% of the US population and have a unique history and music, their own vernacular (AAVE), etc. We should seek to preserve these diverse cultural identities, but they are generally lost in families where the parents come from different cultural/racial groups. For example, if the current generation chooses mates without regard to race then in the next generation only 1.74% of offspring will have two African American parents. In the following generation the number drops to 0.03%. The entire third generation would fit into a typical school auditorium and the fourth generation would be extinct. The same would be true of every other unique culture in the country in a single-digit number of generations.
Am I now supposed to be shunned for making that argument? What part of it is even logically invalid? You can disagree with it on the basis of priorities. I disagree with it. But why is a person making that argument a bad person? Why should that line of discourse be verboten? If you think the argument is wrong then say why. If you have to run the people you disagree with out of town because you can't refute their positions, chances are you're the bad guy.
The way I see it the state does not have a say in who I fall in love with. Neither do you or anyone else.
How about we replace gay marriage with Christian? I have found that in the same circles that want acceptance and open thinking start to sing a different tune when it comes to religion.