Primary Headache Associated with Sexual Activity: This condition is characterized by headaches that occur during or after sexual activity, including orgasm. It is a recognized entity in the International Classification of Headache Disorders and can be considered the most likely diagnosis given the specific context of the headache occurrence.

Migraine: Although migraines are more common in younger populations, they can occur at any age. The headache during orgasm could be a migraine trigger, and given the prevalence of migraines, this diagnosis should be considered.
Tension-Type Headache: These headaches can be triggered by various factors, including stress or physical exertion, which might be associated with sexual activity.
Hypertension: Uncontrolled high blood pressure can lead to headaches, and sexual activity can transiently increase blood pressure, potentially triggering a headache in someone with hypertension.

Subarachnoid Hemorrhage: Although rare, a sudden, severe headache (often described as "the worst headache of my life") can be a symptom of a subarachnoid hemorrhage, which is a life-threatening condition requiring immediate medical attention. The association with sexual activity is known, as it can increase blood pressure.
Reversible Cerebral Vasoconstriction Syndrome (RCVS): This condition can cause severe headaches and is sometimes triggered by sexual activity. It's crucial to diagnose and treat promptly to avoid complications.
Pituitary Apoplexy: A sudden headache during orgasm could be a symptom of pituitary apoplexy, especially if accompanied by other symptoms like visual disturbances or acute hormonal deficiencies. This is a medical emergency.

Arteriovenous Malformation (AVM): While rare, an AVM could potentially cause headaches during sexual activity due to increased blood flow and pressure. Diagnosis typically requires imaging studies.
Cervicogenic Headache: These headaches are referred from the cervical spine and could potentially be triggered by the physical aspects of sexual activity, although they are less commonly associated with orgasm specifically.