Hi again Gary,
What this refers to is the fact that fertile cervical fluid is only present in the woman's body for a few days per month, ie, during her fertile days. Therefore, what Toni Weschler is saying is that if sperm have entered a woman's body at a time when she is not fertile (ie, is not producing any fertile cervical fluid) then the sperm will die after a few hours. On the contrary, if they entered when she was fertile, the woman would almost certainly have fertile cervical fluid present and the sperm would live longer, ie, for a few days as opposed to a few hours. On a related note, a woman gets other kinds of cervical fluid at various stages during her cycle, but none of these are of the fertile type.
I'm afraid I don't know in detail the answer as to why sperm die anyway after a few days - anybody else have any ideas?
Best wishes,
Laura B