Look I know we are all cool antinatalists here on mangadex, buuut
I don't think it was specifically said that she didn't want kids, just that she didn't like them. I also never got the impression that she was against having children, just that she didn't feel confident with interacting with them.
I agree that it was shitty of the boyfriend to not have a conversation about this beforehand, but I also think that this would probably be a touchy subject between them. Just as he knows she isn't the best with kids, she probably knows that he likes kids and probably wants them. I think from the guy's perspective this was a nice way to do both a solid for his sister and introduce the idea to her in organic way. Was he right, probably not, but I think this kind of situation is honestly pretty realistic for a great deal of couples. People aren't always on the same wavelength, and because of that partners don't always make great choices.
There is definitely a conservative family values bias here, but I don't think this was about Ritsuko not wanting kids, just that she didn't feel ready to have them.