I do all the same things a Mother does and I'm not one. Go figure.
And I think that's the point I was trying to make higher up. I think what makes it very difficult is that the role of a SM is largely defined.
We rarely hear of SF moaning, yet they do face what SM do, but on a daily basis. If anything, they should be the one screaming how undervalued they are. However, I think that in the majority of cases, their role is clear, they do very much act as a second father, if not first. They are usually the main earner and provide support. They don't tend to get as involved in the education of the kids or think further as to how they will grow as adults.
SM though are either expected to take on the full parenting role when the kids are with them, or not get involved at all, and then there is the grey area in between. The reason why it worked ok in my family is because we were all on the same page. I definitely didn't want my kids SM to bring in her principles into raising my kids and that's something I felt strongly about. Thankfully, she had no interest in getting involved to that level and roles were clear between her and my ex that he was responsible for the kids when they were with him, including cleaning, etc...
The conflict comes in when the threesome doesn't work. The couple might be on the same page, but totally at odd with the ex. Sometimes as we read this the two parents are the ones on the same page but not with the SM. And then you do have the times when the women agree, but the issue is with the father happy to delegate all his responsibility to the women involved in his kids' life.
I think the 'you knew what were getting into' relates to this. How many SM get into a relationship knowing whether their expectations/principles/values are aligned with their partner and the ex? Of course they don't have to be with the ex, but if that's the case, then they do get into the relationship with some knowledge that this difference might make their lives harder.
I don't think anyone fully appreciate how hard it is to start, build and maintain a relationship when children are already in the picture but it doesn't take away the fact that we are all responsible for the choices we make and how we make them. I really believe that one big error people make is to rush things assuming any issues will resolve themselves because they will bring changes to the relationship, rather than spending more time testing whether they could cope with the family dynamics before committing to it.