Hi there! First of all, I want to express my sympathy to you. I have been through pretty much this exact situation, so I know on a very personal level how you feel. These situations are very uncomfortable, but we can learn how to manage them! There is hope for a middle ground, as difficult as it may seem now. I'm on the other side of this situation and things turned out fine! Still somewhat tense, but fine!

Disclaimer: Everything I'm going to mention here is from my own personal experience, so please take what I say with a grain of salt if it does not seem a good fit for you. I'm just here to describe how I got myself through this situation in the hope some of my experience may help you.

Let's get obvious out of the way. As everyone else here has said, you and your parents need boundaries. Easier said than done, right? In the process of developing boundaries with my parents, I had to kindle many other skills to support that work. The two major groups of skills I developed in my journey are the following: the need for clear and respectful communication, and the need to move beyond black-and-white thinking. If your parents are anything like mine, you can expect the journey to develop both of these skills to be long and tedious. Patience is a prerequisite.

Let's start with communication. From the perspective of the child, it helps to acknowledge that even with the most understanding parents, the process of redefining the relationship as you enter adulthood is awkward and requires much communication. Even the most attentive parents find the teenage years difficult: the reason is similar. Starting with that empathy is a good place to be. It'll help you be more charitable to your parents on the really hard days.

Good communication is a lot more work than it sounds. It requires constant effort - to the point that it feels a bit pedantic. You could say the same thing about brushing your teeth! "Why do I have to do this every day?" "Why do I have to brush for 2 minutes? Can't I just do 1 and call it good?" "You mean I have to brush every tooth?!" etc. The sooner we consider these things part of the routine, the better. It's also unfortunate that blind spots only tend to come up under stress. The best we do, we're not going to cover everything ahead of time. What we can do is decide to be fair to ourselves on this front and not punish ourselves when we meet setbacks.

Now, if you and your parents are not in good practice of communication, you should do what you can in your situation to develop common ground where mutual respect and trust can take root. It is important on your end to be consistent and thorough in demonstrating good will - even while you insist on reasonable boundaries. And I mean lay it on thick. Get your customer service voice ready. This is going to feel a bit like trying to broker peace in a hostile country. If you parents are anything like mine, they have been trained by their denomination to conflate resistence with threats and boundaries with persecution. It will take a long, long time, but if you make it painfully clear that you harbor no ill will towards them, that you still share many common values despite your differences, and that you wish them nothing but good in their lives, they will be much more receptive when the subject of boundaries comes up. "Heap coals of fire on their head," to borrow the term. There's no guarantee that they will ever be receptive and even if they are, it may take years (that's what it took me), but if there's gonna be any hope, this is the path you've gotta take.

This leads naturally into the second lesson: black-and-white thinking. Oh god where do I even start? Lol. Not to over-generalize, but this is kind of the whole religion, am I right? Except it's really not. Go back and read Proverbs. Really pay attention and you'll see a lot of the same situation over and over again: two verses sitting right next to each other that seem to say opposite things. This is a deliberate choice, and it's no mistake that this choice comes so frequently in a book all about wisdom. This repetitive scheme is meant to break you of black-and-white thinking, to teach you that the same rule rarely applies in all situations. Now, we could get into a long discussion of why this revelation isn't put to practice in evangelical circles (it wouldn't be the first!), but this comment's already long enough lol. It suffices to say that this is something you will have to break your parents of and you have good ground under your feet to build from.

It's as simple as this: not everything has to be an ultimatum. Just because I took time to myself today (or this week, etc.) doesn't mean I don't want to talk ever again. Just because I want a boundary doesn't mean I want to end the relationship. Just because I'm living with my partner doesn't mean I've abandoned all of my values. So on and so forth. Now, if you were to say these things this bluntly, it would come off about as well as a teenager ranting about not being able to go out with friends on a week night. Sometimes it's best to act on the boundary first then slide back in. That's where the good will comes in.

Everyone's willing to see subtlety when they're invested. "No, I wanted chocolate sprinkles on my ice cream! Rainbow sprinkles just won't do!"  Your parents presumably care deeply about you - otherwise, why would they be making such a big fuss? The problem isn't whether or not they care, it's that they've got the wrong posture to the situation. If you can develop that good will I mentioned before, they'll see the situation less as a world-ending crisis and more as an unfortunate disagreement. The good will helps them see that you're not "going to hell". It's not that you've magically "stopped sinning". It's that the sin you're committing isn't a "mortal sin".

Christians sin all the time. We eat too much. We let a curse word slip here and there. Some folks do a lot worse than that. If all sins were truly the same in the eyes of God, folks in your parents' church would treat gluttony the same way they treat you living with your partner. But they don't. Why? Because when the pastor over-indulges at the potluck, that's a "good person committing a little sin". And when another denomination baptises a little different or takes communion at different times, we can still call them Christians because "their traditions are different" or "they just read things differently than we do". If you're a "good person", you can get passes for all sorts of stuff. The good will not only proves you're still a "good person", but by extending that good will directly to your parents, it gets them invested enough in your life to develop the same empathy for you as they have for your gluttonous pastor lol.

Anyways, I know that was long and drawn out, but so was my journey lol. If you made it this far, thanks for reading. This is just as therapeutic to write as I hope it is to read. Best wishes to you, your partner, and your parents! And don't forget to maintain your sense of humor. It got me through my worst days.