I became polyamorous because my partner was and wanted to open up our relationship. I'll admit I struggled with it at first but did so consensually and after a while I realized I loved it and never want to go back to being monogamous. Well, my family found out and now they are all convinced that I'm being abused and manipulated by my partner who introduced me to the idea. I've expressed myself as clearly as I possibly can that I agreed to it and love the lifestyle but they still treat my partner suspiciously/accusingly and it's making life really hard. I don't want to lose my family and I appreciate they care but it really messes with my head and makes me question my own sanity which isn't fair or helpful, they talk like they need to rescue me from a cult or something. Establishing distance seems to only make their concern worse and more stalker-y. What would you suggest?
This is a tough one to answer, because I don't have any perspective on your relationship. So I'll try to answer in two parts:
If you're confident that your relationship is not abusive, then your family is just being judgmental and ignorant, and it's okay for you to take distance. I have personally had this experience - once, an ob-gyn I was seeing insisted on pressing domestic violence pamphlets into my hand when she found out I had multiple partners. She was very concerned and kept telling me that "men will twist your head to get you to think that this is okay," which is funny because I was the initiator of polyamory with my male partner ten years ago. There are people whose partners sleep around on them and employ abusive tactics to get them to accept it, but you and I are not those people!
It's okay to tell your family "it's rude, alienating, and cruel of you to continue to act and talk like my partner is abusing me. My relationship is one of consent and love, and if you can't accept that, then for my own health I need to take some distance." And then do that. Surround yourself with people who understand you and lift you up. Sure, taking distance makes your family more concerned, but it sounds like they're committed to their warped worldview and they're just going to up the emotional pressure if you try to get out from under it. Making you "question your sanity" is a nasty tactic and anyone who makes you feel like that is awful. Let them be concerned and wrong over there. Far away from you.
If, however - and this seems very unlikely based on the language in your letter, but - if you have any thoughts in the back of your mind that they might be right, if there's a chance that those conflicted thoughts that bubble up when they put the pressure on could be meaningful - check in with friends, other people in the polyamorous community, read up on healthy polyamory. Sometimes families are just bigoted and manipulative. But sometimes, if a ton of people are flagging your partner as abusive, it's worth thinking about a bit more deeply. Sometimes we are too deep in things to really see the whole story. I know I've been in abusive relationships where all my friends were telling me that he was no good, that I should leave him, etc. but I had a hard time hearing them. They could see from the outside what I was in too deep to see.
Polyamory isn't abusive, but polyamorous people can be abusive. If that sounds at all possible, just check in with yourself about it. You might realize that no, your family is just getting you all twisted up with their own nonsense. In that case, loop back to part one. Take some distance and work to build a healthy, affirming polyamorous community around yourself.