Something else to consider: Person X believes that gay sex, or maybe abortions, are immoral. Is it okay for him to show people pictures of those activities in order to shock them? As in the vegetarian example, the imagery adds "information".
But I would certainly classify trying to get someone to oppose gay sex by forcing them to confront how disgusting it looks (to non-gay people) as "immoral mind-hijacking". It doesn't add meaningful information. Technically, you could claim that someone wasn't aware of how disgusting it is and now that they know, that's new information, but that's not the type of information that's normally relevant to rational arguments.
You could also say "well, some disgust isn't rational, but the disgusting aspect of animal slaughter is a different kind of disgust which is rational", but you'd be hard pressed to come up with an argument for why the two kinds of disgust are different that doesn't amount to a direct argument for vegetarianism that could be expressed without involving disgust at all.
In the case of gay sex and abortions, the disgust is the person's emotional response to something that isn't the most important (to the image provider, and perhaps to the providee too) part of the action. Dicks in butts (because anti-gay people always talk about gay men, never lesbians (probably because if they did they wouldn't get the desired response)) are less important to them than adults being able to do something they really want to do with their own bodies. Tiny bloody fetuses having been killed aren't as important as people's lives not being ruine...
I notice that a large fraction of Effective Altruism people are vegetarian. This makes sense: in general Effective Altruists take moral issues seriously, even when that means changing your lifestyle. I'm not sure it's a good balance, though.
One way to think about this is to convert it into money. How much would I need to be paid to give up eating meat? All animal products? How much money would I need to spend on myself to be about as happy as I would be with less money but continuing to eat animals? I'd probably be willing to go vegetarian for about $500/year, vegan for maybe $2000/year.
It turns out you can probably pay to convince other people to be vegetarian much more cheaply than that. I estimate the cost of a vegetarian-year at $4.29 to $536 while Brian Tomasik estimates $11 with better methodology (which I looked at). This is by placing ads on facebook for a site where people can watch an animial cruelty video and ideally become vegetarian or vegan.
If you would get more than $11/year worth of enjoyment out of continuing to eat meat, why not give $11/year to convince someone else to not eat meat for you? Or give $50/year and be on the safe side?
(While you're giving money, you should probably give it to the organization that you think will do the most good with it, which I think is probably one of GiveWell's top charities. The nice thing about money as opposed to actions is that it's easy to redirect.)
I also posted this on my blog.