They are on the AlternateHistory.com message board, so it's not unheard of for a reasonable level-headed forum ideological echo chambers to enact such a rule.
edited for a better description.
But the objection to anti-SSM opinions are in my opinion okay on AH.com, because it's a historical forum where they sometime discuss politics. But what you suggest are making a mainstream political opinion ground for banning on a political forum. It would destroy debate, create a discussion culture of baiting, both people careful avoiding saying anything direct, and also people trying to lay trap and tattling to the moderators. It would be a disaster for the forum.
The moment it was no longer mainstreamed (as in social acceptable to support).