[JustSayNoToNannies]

Kirby showed that acutely stressed rats showed increased RFRP levels for several hours, but that levels returned to normal by the next day. [...] "It's very adaptive to not be wasting resources on reproduction during times of acute stress, to just shut down reproduction for 24 hours or so until the stress is gone," said co-author Daniela Kaufer

The victim may already have ovulated - or may ovulate later and still have in her body the rapist's sperm, which can live for several days. Scant protection there.