Morrigan, no disrespect intended to you and your situation, which sounds difficult. But I don't think an awful lot of sane, rational adults will universally agree with
this bit. Nor that
this bit must always be the case as it's just not always so all the time, even in many first marriages. Often, the kids' needs will take priority for many years because they're kids...especially timewise when younger but also emotionally as they get older and need guidance and wisdom. My son talks to me all the time about his concerns, and that interaction and guidance is as much a part of parenting as feeding and clothing him. It's a different kind of priority, but it's still very much a priority and one I would not compromise on, if someone were mistreating him or expected him to go quietly without having his voice heard and honoured. These really are very brutal and heavy-handed sorts of assumptions and tactics IMO.
If I were intimate with someone who treated my kids poorly then I'd have to question much more about myself than just my sanity. Of course, if my kids were just blatantly respectful to my partner, that's also not acceptable...but if I put them in a bad situation and they are just venting or frustrated...well, I've made my bed and now I've got to work with all hands on deck to navigate a safe passage. It's not simply "the kids' fault" IMO...it's as much mine and whomever else's, as we've sometimes had to amicably break up marriages to free ourselves to re-partner...and sometimes we've done so not nearly so amicably at all. In either case, the fallout often lands on the kids and we as adults must be, well...the adults
I.e., compassionate and kind and forgiving, ideally.
For an awful lot of folks, whether in a first marriage or a 2nd (etc), the kids do come first when push comes to shove. Meaning, especially when their wellbeing is at stake. If that is not recognised as a priority then there is something wrong with the entire situation IMO. There is certainly no mandate that your partner must come first when he or she is mistreating the kids (???), even if short of full-on abuse. No offense, but that's a pretty strange and unpalatable "law" or rule...and I don't see many I have known following it.
Although the parents (natural or step) are primarily responsible, I don't see anything gelling without all parties being respected and heard. I'd follow my own wisdom and seek a family mediation which honours all parties, and if my partner wasn't down with that, then he'd not be partner for long. If the step parent is Mother Theresa and the kids are really vile and it's totally 100% black and white, then family mediation is still crucial, so that the step parent can also be heard alongside the kids. Again, here it's especially important to look at their feelings and allow them to voice and honour their feelings, even if you disagree. How else can you ask for respect if you do not also give it and thus model it and set the example?
Once again, no disrespect intended, but I disagree that a bad family situation for whatever reason means your partner is king or queen and should rightfully expect that your kids be kerbed and "put in their place". That all sounds pretty horrid really and I can't imagine wanting to fight for any of that, really.
Peace & blessings
7L