The deed victim cannot attest to the rapein your example - so there must be corroboration from another witness. To say that sperm is needed is NOT accurate. Sperm proves sex - but it doesn't resolve whether the victim consented or not - which is a crucial issue in rape cases. Any degree of penetration (by penis) is rape - so of course sperm is NOT crucial (not in UK law at any rate). CONSENT - or clear lack of consent is the ultimate evidence in a rape case, whether that is through a witness, CCTV or other testimony (recording).