AITJ for not wanting to Breastfeed My Stepdaughter?

I am 7 months pregnant with my first child. My boyfriend (of 5 years) has two daughters 8f and 0f. His second daughter was conceived while we were on a break. I don't really have a lot of family to teach me about babies and child care so I have been learning online a lot and also attended a couple of classes being held in my area. One of the things I learnt was the importance of breastfeeding and breast milk. I think it's such a wonderful thing to be able to offer that to my child when he is born because it has life long benefits. A few days ago my boyfriend asked me if I had twins would I formula feed or naturally. I told him I would prefer to breastfeed and if there wasn't enough then I would supplement with formula. Then he told me he would want me to give my extra supply to his newborn daughter. He even said that since I am already lactating I can start pumping now to give it to her. I said I was not comfortable with that but he started to guilt me by telling me that it's not fair for his child to not get the amazing benefits that breastmilk has to offer. I understand it's not fair that she doesn't have a present mother but my boyfriend and I had agreed that I will be like a good friend to both his daughters and that their parenting is not my responsibility. He was also upset because in the past I have donated blood which goes to strangers but I won't do this for his child. I would like to clarify I only donated about 3 or 4 times and stopped because I wouldn't feel great after due to low iron. So in a way I prioritized my health first over donating to others.

On one hand I understand that he wants the best for his daughter but on the other hand it's my body. Maybe I would be comfortable donating breast milk in the future but not anytime soon. I would want my own child fully fed and weaned off first and I would ideally only wanna donate after I'm done having all my kids (2 or 3). I also dont want to be feeding more than one baby because I want to go back to my pre pregnancy body ASAP and have the least amount to stretch marks and deflated/sagginess.

AITA for only wanting to breastfeed my own child?

PS he also said if I breastfeed his daughter, she is more likely to look like me and feel less insecure about her body. He doesn't want his younger child to be overweight like his older child. He says she feels bad that the tops I wore pre-pregnancy are the same size as his 8 year old would wear. I told him that her and his weight issues can likely be resolver with a good diet plan and that genes don't work like that? (I might be wrong though). He also said if I refuse to give the extra milk to his daughter then he doesn't want me wearing Brandy Melville because that's my favorite store but he claims its toxic.

EDIT: I WAS NOT AWARE OF 0F UNTIL ABOUT A 1.5 MONTH AGO. OBVIOUSLY I'M NOT STUPID ENOUGH TO KNOW THAT AND STILL GET PREGNANT

EDIT 2: Baby's mother is a drug addict, not present in baby's life. 8fs mother has her weekends only, she has a bit of a drug problem, mainly Marijuana