A guy goes to a travel agent and books a two-week cruise for himself and his girlfriend. A couple days before the cruise, the travel agent calls and says the cruise has been canceled, but he can get them on a three-day cruise instead.
The guy says "OK," and goes to the pharmacy to buy three Dramamine and three condoms.
Next day, the agent calls back and says he now can book a five-day cruise. The guy says he'll take it.
He returns to the same pharmacy and buys two more Dramamine and two more condoms.
The following day, the travel agent calls again and says he can now book an eight-day cruise.
The guy says, "OK," and goes back to the drug store and asks for three more Dramamine and three more condoms.
Finally, the pharmacist asks, "Look, if having sex makes you sick, how come you keep doing it?"