What if they became pregnant?

I've thought of that, but I can see why such an arrangement wouldn't work. Part of the trick in this whole arrangement, and what makes the film most interesting on a human level, is that these "donors" all very dutifully accept their fate. There is never any suggestion of any resisting, saying no, running away and hiding, or escaping the UK altogether. Part of their upbringing must involve the conditioning to make them passive and accepting of their fate. If they could have children, that give them an incentive, even an imperative, to live on. They would want to care for the children, see them grow up, maybe even escape the sad fate they have. Part of the trick is to make their lives so different from ordinary humans that they accept their fates.