Greentext

Firstly, it IS the DM's choice what setting they choose. If they want to mesh a realistic medieval aristocracy with dragons, wizards, etc., then that's their decision. If you don't like a DM's world-building choices, you can always take your dice to another table.

Unless they DM is discriminating against the actual players, then they're entitled to tell whatever story they like - and its up to the players if they want to sit there and play it out.

You're acting like the DM stood with their head high holding a straight pride flag or something while shouting at the player for even daring to ask about gay marriage, rather than clarifying the marriage system of their setting.

Secondly, again, if the DM was truly a homophobic bigot as you keep saying - do you think they'd have continued going along with the player's conquest to legalize gay marriage in the kingdom?

If they were truly a bigot and hated the idea of legalized gay marriage in their setting, they are equally as entitled to walk away from the table as all the other players are. Yet they stayed, narrated, and played out the whole thing - even though it meant watching their campaign essentially be overridden by something they weren't as interested in.

Expecting a DM to build their setting based around what you think is acceptable, and calling them bigoted if they don't, comes across as entitled to me (which is ironic considering you're the one talking about red-flags).