One thing that bothers me about the Red Room sterilisation theory is that they most likely did a tubal ligation (anything more than that and you risk issues like early menopause, even if you leave the ovaries in place) and there is no reason why someone with tubal ligation can't have children, with a bit of help from reproductive medicine to collect eggs. I mean, I can understand if Natasha didn't want to go near doctors, but there's so many options even for her to have biological children if that's a thing that was relevant to her.