Without getting too graphic, Sumthnwitty made a good point about super-charged immune systems. With conception the sperm is actually seen by the woman's body as a foreign object and defense agents set to work against the sperm "invasion". It helps prevent more than one sperm fertilizing the egg and creating problems, so there might be something too good about the Others immune system that keeps them cancer-free, but also keeps them sterile. (I hope I explained that well enough without being too detailed for some)

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`Dear, dear! How queer everything is to-day! And yesterday things went on just as usual. I wonder if I've been changed in the night? Let me think: was I the same when I got up this morning? I almost think I can remember feeling a little different. But if I'm not the same, the next question is, Who in the world am I? Ah, that's the great puzzle!'

-- Alice, from “Alice’s Adventure’s in Wonderland”