The problem here is that you are making a whole load of assumptions about a woman that you don't know at all.
You are criticising her for getting pregnant too early in the relationship, attributing emotions to her, suggesting that she is pleased with herself. How do you know all of these things? For all you know she could be worried sick and wondering how she will cope.
I fell pregnant quite early in my relationship with DH. We already both had children, and I had been told I was infertile after various medical issues. Somehow I fell pregnant, and despite it turning our world upside down, it was wonderful for all of us!
I wonder what my ex/DH's ex may have initially assumed about the whole situation? I don't really concern myself about it, and they never created issues for us.
None of our children are damaged, and nobody has had to consult anybody else about pregnancy plans. If you fell pregnant now, that would be fine too. It's not a race, and all babies are different.
Babies are lovely, and children love their younger siblings, despite reservations that they may have prior to their arrival. One of mine was very worried, but they adore one another now.
Your job, as their mother, is to provide reassurance that their Dad/you will still love them when the new baby comes along.
You can hate him all you like, but you mustn't try and indoctrinate your children, whatever your feelings and justifications may be.
Kids figure things out for themselves in the long run. You have to stay neutral with them, and love and support them unconditionally.
You never know, you may one day find yourself having fond feelings for this baby, once it has bonded with your children.
Stranger things have happened!