[Valpal1]

Women are not in all cases allergic to semen itself, but to chemicals in the semen from food, beverages or medications the man has had, ranging from penicillin to compounds in walnuts. However, "this is rare, even more rare than semen allergy itself," Resnick said.

Um, no it's not. It's an unpleasant side effect of many antibiotics, they may also give ladies a raging yeast infection. Make him wear a condom until 3 days or so after he's finished the antibiotics.

[Fierce Allegiance]

An episode of House dealt with soemthing similar a few weeks back, IIRC.