Of course it makes sense, are you being deliberately obtuse?
I will give you the benefit of the doubt and explain what I'm saying.
When a woman meets somebody she is fond of, and they happen to have children, almost all of those women proceed with open arms and the best of intentions. They know there is a child/children and intend to foster a loving relationship. They have no problem with there being an existing child.
It is not until later on, after they've already committed to the relationship, that the issues start to arise.
Case in point, the OP's.
The child's mum was very respectful and decent with the OP to begin with, so why on earth would it occur to the OP that she would be anything but that, later on?
Coincidentally, in my own situation, it was also my pregnancy that sparked something with the ex which lead to her doing a 180 and causing a ton of grief.
It's likely insecurity on their part because they're worried about their child being replaced, but they act out that worry in the most childish of ways, see - refusing holidays, being difficult with arrangements, that eventually all of that leads to the (again, well intended) step mother becoming resentful over time and thinking you know what, fuck that.
So you don't know what you're getting into, not really, dynamics change over time and you can't always (or ever actually) predict how other people are going to behave.
As an aside, I hope those wielding the pitchforks know that piling on to an OP like this is going to have the opposite effect to the one they're hoping for.
I came on mumsnet asking for advice and a bit of a chat when I had DSC troubles. I was very open to advice and happy to listen to anybody who felt IWBU but after pages of a pile on giving me nothing but grief for daring to take issue with my disabled child being assaulted, I thought sod this - fuck this, that lot on here and the DSS included.
There is a reason counsellors don't give their opinions, they give people a place to get things off their chest. It's not beneficial nor helpful to tell somebody what an arsehole they are when they start to open up about something difficult in their life.
The OP needs to work through her feelings, all this thread will have done is brought the walls up and reinforced the opinion that her and her DC don't matter. Are you really surprised that she has doubled down the more the thread goes on? That was the only outcome.
Own goal for the ex wives club.