"Coital cephalalgia: Also known as "sexual headaches", coital cephalalgia is a rare type of severe headache that occurs at the base of the skull before orgasm during sexual activity, including masturbation. The pain usually moves from the base of the skull through the head towards the frontal lobes. Extremely severe and sharp pain behind the eyes is also a symptom. The headaches usually have an immediate onset, with some gradually worsening during intercourse and others (referred to as "explosive headaches") occurring almost instantaneously at the moment of orgasm. These headaches typically last for a few minutes to a few hours, although it is possible for such headaches to last up to a few days."
If that isn't enough to make an atheist out of you, I don't know what is.