Let me try to make sure I understand. Are you saying that the theory is that because of the Siphoning, which may have somehow activated Spike's semen to being fast little swimmers again, happened a few days after the party at which Buffy conceived her child or that she didn't actually conceive the child until five days later? I have to admit that I'm very grossed out to imagine Spike's non-motile sperm staying in Buffy's area that longer and that Buffy didn't use any feminine hygiene products (or Plan B, for that matter) after the party. Although, since this is probably a mystical miracle baby, Plan B probably wouldn't have worked.