My (21f) father (57m) is having ivf twins with his new girlfriend (37f) who I still haven't met. He didn't tell me until she was four months pregnant.

Edit 2: don't think this is relevant but realised 20 weeks = 5 months. I'm dumb
Edit: A lot of people seem to be focusing on the ivf aspect which is not surprising, I have no issue with ivf. Nor would I have issue if my dad hadn't denied he wanted children and reassured me that it wasn't an option. I know 4 months is a landmark in terms of knowing whether a baby will take in ivf. I understand why couples want to wait until they are sure. But this is so out of the blue, he never discussed this was a possibility. He reassured me it wasn't.
A lot of comments also seem to be telling me to basically get over it because its not my business. To be clear i don't feel that my dad needs to ask my permission. But I think going from being told this wasn't a possibility to being told my dad is going from a 57 year old semi retired man to raising twin babies is pretty ****ed. I have no expectations that he should leave this women and not raise the kids. I think they should be his priority of course, and I hope he gets his **** together for them.
I'm just trying to comprehend what has happened, and how this will change things for me. I appreciate your help Dating.mobi xxx

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First I will just give a bit of background. My parents have always seemed to hate each other. There has been lots of cheating on both sides and they have never been affectionate or loving or even friendly. Things came to a head when I turned 18. There was a particularly bad episode of cheating/fighting where it came out my dad was in a serious relationship with a 35 year old woman who had a 16 year old child. My mum was so angry she set all the Christmas presents on fire. Despite this, she took a year to divorce him - I think she just wanted to see how scared and pathetic he was, which is sort of understandable but it was traumatising for me. Basically, my parents were really ****ed up and having a toxic upbringing has been very detrimental to my mental health, I have dropped out of uni as a result of quite severe OCD etcetera.
So they eventually separated last year when I was 20. I still hadn't met dads girlfriend even though by this point they had apparently been together for about two years (I think), I found his and mums relationship so awful that I just didn't feel ready. Also, his girlfriend cheated with my dad on my mum so I wasn't exactly open to the idea. Also my dad would often work away from home about 4 nights a week and leave me with my mum who definitely has something akin to bpd, which I used to understand because he had to earn money. But then I gradually realised that actually he was probably with his girlfriend a lot, which is sort of depressing because he knew how abusive mum could be.
So fast forward to now, dad tells me he's moving in with his girlfriend and child and stopping the lease on our flat. I am initially hurt, but I can go back to uni accommodation soon (living with mum isn't something I want to do) so it's not like I'd be homeless. He also offered me a room in his new house, saying he hopes I have a safe space with him. After some thought I kindly tell him that I will think about it but I'm really unsure and I want to take everything slowly. He seemed very respectful.
A couple of weeks later I decide to try and find out more about his girlfriend through Facebook and discover that she is 20 years younger than him. That really freaked me out because she is closer in age to me than him. I think it pushed me to realise actually I am still not ready to meet her and her child as everything just felt too ****ed up and complicated. I wrote him a calm email detailing why I felt like that, and he said we'd talk about it in the evening.
Well, last night he told me that she was four months pregnant with donor sperm ivf twins, and that he will raise them as his own. Apparently he has known his girlfriend wanted to do this their whole relationship. He said he didn't want to tell me because he thought it would stress me out and make my mental health worse, the patronising idiot. I just sat there getting angrier and angrier. He was an absent father, a serial cheater and just generally not the best dad. I have tried hard to see past this and we meet up regularly for nice lunches and walks etc. But four months. Twins. At 57. He tells me like it's not a big deal. I was so so so angry. I feel so disrespected and just let down. He said I am his number one priority but clearly it's just not true. He said if would be "really sad if we became estranged over this". I just sat there with my mouth open. What the **** do I do Dating.mobi?
TLDR my dad is having twins with his girlfriend I haven't met


Source.