I already know that part if not all of what I'm going to say is bad etiquette, but that's not really the point- among many other things, we're not offended when someone addresses a letter using Master/Mister wrong, even if it's improper etiquette.
So, if you're invited by someone who doesn't know the person you're dating, and they don't give you a plus one, are you offended?
Does it matter how long you've been dating? Is there a line in the sand about engaged vs dating?
Different if you're married? (and is that different from living together?)
What if you're not seeing anyone and they don't want you to bring a plus one?
Is there leeway regarding how many people you know at the wedding, or how many people are there?
(^-- Most people here are married or getting married, but if you think there would be a difference, just imagine it)
This didn't come up at my wedding, but it has come up now and then otherwise, see comment.