Some people react to proteins in a partner’s semen with itching, burning, swelling, redness, hives, or vaginal pain after sex—called seminal plasma hypersensitivity (semen allergy). Reactions range from local irritation to rare systemic allergy. Diagnosis uses medical history, allergy tests, and sometimes supervised exposure. Management: condoms, immediate genital washing, antihistamines or topical steroids for mild cases, and allergy shots/desensitization for severe cases. If you get facial or throat swelling, trouble breathing, or dizziness after exposure, seek emergency care immediately.