I've been a "SM" (wasn't married to the father, but had long term relationships) twice.
The second instance was great. 3dc from his previous relationship. All lovely children. I wasn't mum. I was dad's partner. Because that's what I, DP and ExW all accepted. No one pushed me in a box, no one said out loud, you know Courtney isn't your mum, and it worked. Really well. 5 years on, they occasionally message me, because, well, we like each other. I'm a nice person. They are nice children, who are now nice young adults. DP handled everything so well between all parties, that it just worked. I truly believe this is because DP and ExW mutually separated. There was no "who's this b*tch living what my relationship should have been" unwarranted bile in the background.
The first instance was horrendous, and it was categorically down to the ExW in that situation. The audacity of DP to have left her. The cruelty of breaking up her home (they were never married either, if it makes any difference) which was only ever viewed from her scorned viewpoint, and never from the children's. The children were continually upset by her "we don't have a family because daddy lives with Courtney now" despite me not meeting him until long after he had left her. She sent her DC round with secret note books to write down and report back what me and DP were doing, we found them in their room with pointless entries such as "daddy is making sandwiches and Courtney has gone to Tesco. 1.15pm". It was all so twisted and weird. Endless "If daddy really loved you, he'd live with you, but he wants to live with Courtney doesn't he". They were brainwashed beyond belief, all from her woman scorned perspective. I was still the same person who happened to go on to have a great SM relationship with three children in my next relationship, so I know it wasn't me. To be honest, credit to that awful woman's children, because given the nastiness they were constantly fed, they took a lot in their stride and did enjoy quality time with their father. It was a large factor in me ending that relationship. It's now nearly a decade on, and I know he's not been in a relationship since me. She will have been there at every step making sure of that.
I'm long past both of those situations, and happily married now, but having experienced it myself, and seen many friends in very similar situations, the foundation of any relationship absolutely lies with the ExW. If she is a decent woman, she will understand that and build a strong foundation for her children. If she's not, she will capitalise on the power she has, and ruins it for everyone, so she wins, even as her children lose.
I think it's actually a crying shame for some SM. To find someone they know they would have happily spent the rest of their life with, to find their soulmate, and to lose that, by the endless manipulation and sabotage from a woman scorned in the background, with the smugness that she will always be in the background.
Of course, some SM are arseholes too. Absolutely. But it seems to be the unspoken elephant in the room that in the majority of cases the ExW feels bitter and that's the root of everyone's problem.