Yeah–the player that used it didn’t actually say “X card,” he said something like “Play a nope card,” or “I’m gonna nope that.”
There’s “not fun” and then there’s “not fun.” If a player is taking too long with their turn, or using a goblin voice that I find grating, or looking at their phone during the scene description, or slurping their coffee, or refusing to speak in character, all that might make the game less fun for me–but it’d be a dick move to X-card that, IMO.
But in this case, murdering an innocent bystander simply in a fit of pique instantly makes that PC a sociopathic villain, and our lighthearted romp of a game becomes something very different. If we don’t immediately turn on that PC, dropping everything to stop him and abandoning our mission, we’re complicit in a gruesome sex murder and mutilation. No phobia or PTSD is involved, but it turns the last couple of hours into laughing with a bunch of friends into “Jesus, what have we become?”
I think gdave’s example is pretty cool. For myself, I can totally see playing a scenario where we kill a bunch of evil US soldiers and don’t think twice about it, and I’d not even think about whether some other player would take it personally. But if I found out we’d played through the scenario and it made a player feel shitty, I’d really regret that. Being able to say, “Hold up, y’all, this isn’t fun for me” would be important there, and I’d be 100% willing to change the game to keep it fun.
Similarly, I have a player in my regular game who freaking hates the idea of fighting animals. Killing animals makes it so un-fun for her that even a giant undead squid is a little on the squicky side. I’ve adjusted my scenarios so there aren’t times when straight up animals are combat enemies–or at least, there are alternatives to killing them. No worries; now that I know this, I can adjust.