My absolute hard boundary is physical violence directed towards me or another living thing (other than, say, insects ?). I grew up with a door slammer and thrower (my dad), and he never directed it at someone else, human or animal, but it was terrifying enough. I'm also inclined to not tolerate verbal violence directed at me or someone else (IE screaming AT someone), though that's a bit more flexible for me, and situation dependent to an extent. Kinda depends on how threatening it feels.
I also have low tolerance for cheating, however that is defined in the current relationship. IE, even in an open relationship (not with my sufferer, this was long before him), we still communicated what was going on on the side; cheating for us was dishonesty and sneaking around with someone and not telling the other person. He still managed to cheat by lying about who he was having sex with, and when (he told me he wasn't having sex with anyone including me, when he had a couple of people who came around pretty much whenever I wasn't). For me, (situation dependent, of course), I find out the why behind it, and figure out if we can move through it. Second instance of cheating? Nope, done, hard boundary.
A lack of physical intimacy for me is also a deal breaker, and one I allowed to fall away with my sufferer. I understand desire ebbs and flows, and that mental illness can take a HUGE toll on intimacy (been there, myself, but realized it was a problem and tried to figure it out - now I know what to watch for). But I KNOW I'm not compatible with someone who can't stand physical touch. I (most likely) wouldn't get involved to begin with, with someone who does not want physical intimacy (and is honest about it). And I know it's probably intolerant of me, but I probably couldn't be in a monogamous relationship with someone who identifies as asexual or non-sexual.
In the case of an established relationship that becomes non-sexual (IE what happened with my sufferer)? A lack of honesty - with me and with themselves - that's the issue. "I want to be intimate and I'm working on it" only means something when the actions match the words. But, this is less of a deal breaker I guess than a "Let's see if we can work it out." Physical and sexual intimacy is difficult, and it's a boundary I'm still working on myself.
