I have different experiences here:
As exw I can see that the new gfs or housemates (ex rents a largish house that he cannot afford alone so there have been various co-tenants) have been told a crock of shit about what an evil bitch I am. To be expected really, so I am just nice and polite to them, which no doubt confuses them. I mostly feel a bit sorry for them, because my ex is really not a very nice person - narc in the extreme. But as individual people I want them to be kind to my kids, so I am nice, kind and generous to them, particularly when they become exs who often want to see my kids after the relationship is over.
Ex has had some extremely dodgy gfs - but that is his problem not mine, and the courts here in France wouldn't give a shiny shit about that frankly.
When we were initially divorced I was worried about other people parenting my kids, but I seem to have got over that. I am more worried about exs ability to parent.
I do not really hold with MN view on partners. People have come and gone in my kids lives, we have lots of people living with us as au pairs etc for up to 2 years and they leave and we miss them, but our lives are richer for knowing them. I'm not convinced that meeting my exs gfs and house mates has done my kids any harm at all. Even the total nutter that was arrested by the police while my kids were there. Not ideal obviously, but no-one got hurt. The damage done to my kids has been done by my ex, and only by him.
As a step-mum, I can see that I will never be my step kids mum, and they don't think of me as some kind of replacement or a second mum. There is no confusion of roles. However, they can see that I adore their Dad, and that we have a really safe relationship, where we are very kind and supportive to each other. We are modelling to all our kids the kind of relationship we would like them to have. Hopefully we are doing a good job.
My DP's ex is a pain in the arse, but is never rude about me. I am less polite about her, because her behaviour has a very direct effect on my life, and indirectly the life of my kids. I'm not sure that exws always think of the effect their behaviour has on the new partners kids. In fact I'm fairly sure that never crosses most exws minds. And maybe why should it?
My DP is a good step-dad, but my eldest has issues, because of her own dad, so it's not always simple. The youngest loathes her Dad, but both of them respect DP, and see that our relationship is solid.
As a step daughter, I am very close to my step mum of 35 years. But again no confusion of roles, and she never treated the 4 us any differently to her kids. We all lived together in one home. I think being fair as a step parent is really the best we can do. We step up to the mark for all of our kids, and we hope the other people in their lives do the same.