A warning up front: I'm about to explain a slightly complicated point. If you wanted something light with your morning coffee, this isn't your read — skipping is allowed and even encouraged.
Back to business: there are no "red lines" in a relationship. There is a red line.
What do I mean?
I keep running into posts that list "the signs" that your relationship is bad for you — inventories of "if she did such-and-such, that's a red line," or "if he doesn't see you and hear you every fifteen seconds, walk away."