I have been in a situation somewhat similar to yours, albeit less extreme.
In my case, it was (mostly) a series of misunderstandings. In the end, everyone talked it out and it is (mostly) okay now, but I want to tell you this because I reacted like you did. I wanted to stay neutral and defend everyone to everyone else and doing that almost broke my marriage. And that was a less extreme case.
I'll tell you what I wish I could go back and tell myself.
Your SO needs to be able to vent to you about your family.
There's a difference between talking shit and pointing out that some habits or behaviors are frustrating. If you are like me, it hurts when someone says your bio family is wrong (even if they are). That's a gut reaction and it can and should be put aside. If you never let your family be wrong, your SO will end up resenting you and hating them.
I had a lot of trouble suppressing my reaction to defend anyone in my family, it's tough because you don't want to check out of the conversation but sometimes it's gonna sting a little. Try really hard to step back a view it logically. The sooner you do, the easier it'll be because I'm guessing you'll be having a lot of conversations about this stuff coming up
Explaining what you think someone 'meant by that' is still defending. If it's super obvious then sure, like if someone apologizes and another party thinks it's a sinister plot, then maybe throw in that you don't think it is. (Unless there's a history of sinister plotting)
Any time you don't think someone can handle the truth, reflect and decide if they actually can't handle it or if you are just afraid to deliver the truth.
By getting married, your SO is your family now. Separate unit. They should be #1. If you can't say that you love them more than your bio family, then you may be rushing in. (Even if you aren't rushing, if you're really family oriented, it still might feel weird but it has to be true)
Keep in mind that this situation probably won't last forever. If you want to have a chance of resolution, tell your SO explicitly that you are on their side and that their feelings are valid. Actually even if you don't care about resolution, tell them that. It's just good to tell your SO that.
The recovery time from my situation was ~2 years. Buckle up and make sure you know where your priorities are.
Good luck!!