@Drunkenmonkey
No, it isn’t even remotely the same thing.
Firstly, using IVF or adopting when single in no way requires duping the other parent into conceiving another child without full knowledge of the fact that once pregnant, their partner is going to leave them. That is pretty despicable behaviour. Can you imagine if a man suggested to his wife that they have another child, whilst simultaneously planning to leave her the minute she was pregnant/the baby was born? He would be called all the names under the sun on MN. Rightly, it’s a vindictive, selfish thing to do. But when a woman does it? Quite a lot of women seem fine with it. In any case, it is in no way similar to choosing to adopt or have IVF as a lone parent. Though I do think the latter of those options can leave unanswered questions regarding their parentage and that is worthy of consideration, even though I might not term it selfish.
Secondly, OP paints her partner as a lazy, reckless gambler who has a short fuse. Someone she dislikes and does not trust. Yet, she wants to saddle another child with that sort of man as a partner. Again, utterly dissimilar to the situations you are talking about.
If he is as bad as op says he is, why does she want her children to be exposed to that? So they can have a sibling? So she can attempt to fulfill some Disney fantasy of happy families skipping off to daddy for the weekend leaving mummy to have a lovely restful weekend, resting assured that they’ll all be having a wonderful time because they have a sibling. Of course, if they did not, their time with daddy would be awful.... but why would that be if he is a wonderful parent? Or is she foolishly trying to recreate her childhood? Big gamble, no guarantee she’ll get the cosy set up she’s hoping for. That whole situation is selfishness personified. Dressed up as for the benefit of the elder child. It might work, she might get children who are get on well, are close and have a lovely relationship. Or she might not.
She might get an angry, resentful ex-husband who will do his best to make life difficult for her- pay minimum he gets away with, take little interest in the children, or worse- drop them when he meets someone else/if they start a family. He may even resent the youngest child, putting a wedge between him/her and the elder child. Of course, this is a worst case scenario and if he is a half way decent father he will step up and do his share of care/parenting and financial support. But.... given the picture painted of him by OP that does not seem the most likely outcome, does it? At the very least I imagine she’s going to have a resentful co-parent, which makes it harder than if it is amicable. Of course, that can happen after any break up, but OP is actively engineering a situation that makes that highly likely.
Being a single parent is not selfish- lots of relationships that were working well when children were conceived go on to fail. Lots of women cal pregnant outside of a serious relationship and go on to be good mothers with happy children. That is not selfish. If a single woman can financially and emotionally support an adoptive child, or her own child conceived by IVF , that is not selfish. What is selfish is tricking another person into having a child with you without knowing you plan on ending your relationship (I.e. they will also become a single parent to 2 children) so that you can have full siblings (so avoiding social embarrassment?). Worse still, when you add in that OP paints her DH as a feckless, lazy man with little in the way to recommend him and a very short fuse. ideal father material? Doesn’t sound like it. it’s one thing if you haven’t realised he isn’t the best father, but once you have to do it again seems selfish to me.