The timing on that sort of thing is too sensitive to be left to the vagaries of human emotion, hormonal cycles, and other bits of sexuality. So when the stars are about to be right, the chrysalis impregnates the bearer regardless of whether such things would normally be possible. The Shadowchild was "split" because Charon was already pregnant when hers activated, which complicated things a bit. The Hydra chrysalis, on the other hand, took "properly" because for whatever reason Christmas hadn't become pregnant, despite her and Jackson... doing things that would leave some doubt. This, incidentally, is why the "hosts" of the creatures look so much like their mothers; they are completely unrelated, or related only in passing, to their fathers, as the magic of the chrysalis makes them essentia