It does seem a little over-the-top as a true story, but when you say, "we don't want to host murderers at our wedding" I think you've made your position clear, not just for the wedding, but for your relationship to everyone going forward. Like, you can't say, "We don't want to host murderers at our wedding, but they can come to our birthday party next week." No one's going to say, "Oh, I understand, dear. See you next week!" If you really want to cut ties with omnivore family members, and anyone who supports them, for the rest of your life, okay, that's a pretty clear way to do it. But one often gets the feeling that these people expect everything to go back to normal a month later, and that no one ought to be offended or wish to disassociate from them. And a lot of the time, other family members who are spineless or overly optimistic will keep inviting them to stuff.
I have issues with some of my family members where I might be thinking to myself, "Wow, I really don't want to host that racist at my special party." But however righteous you think your stance is, you have to accept that there are going to be long-term consequences, and decide for yourself if the consequences are worth it to you. I'd rather just not host a party at all, or keep it so extremely small that there are many who aren't invited (including the racists), or graciously invite everyone and react appropriately if they actually do misbehave.
For this story, it kind of sounds like something else might be at play, since people were first invited, then uninvited. You didn't suddenly realize your aunt likes eating hamburgers only after you invited her! So (having not actually read the full story) I am guessing something tweaked the couple after invitations were issued, so they decided to rescind them and try to frame it as a high moral stance, when in fact it was probably about something much pettier (odds-wise). I'd just LOL and drop the couple from my mind--lucky break, really!