The more I think about this the more i think that's it's case of expectation vs reality.
People have lots of fun ideals about what type of parent they will be when they have kids, then reality comes along and gives them a good shake. Sometimes though they cling to those ideals and then their MH takes dive because life isn't turning out as planned, reality isn't what they thought.
There are lots of expectations re blended families and what "should be" but not a consistent pattern of expectations that could be formally agreed upon and a book created. Which is a problem glossed over by many here.
This means that actually when reality rocks up it's not just messing with one perspective but all perspectives.
The closest people who can tell you about the reality of step parenting is actual step parents, the unrealistic expectations are often said like law by people who have 0 grounding in step parenting and it doesn't marry up with reality.
As nice as it would be to claim the title as a mum, I know I'm not a perfect parent. I'm not sure why everyone seems to think SP should be held to a higher bar when any hurt that was created by first family splitting up, is somehow magically and morally now SP problem to solve.
If your a parent I bet your bottom dollar there's one thing you said you would never do as a parent but when parenting reality hit, you changed. That's allowed. So people's views on step parenting allowed to change. Shaming people won't stop those feelings.