I don’t know what the rest of the world does to advertise safe sex, but one of the themes the safe sex groups have used here for a long time is the premise that by sleeping with one person, you don’t know whom else they’ve slept with and what they might have caught.
Taking that a (slightly cynical) step further can you catch other undesirable traits of your partners exes? For instance, if you slept with Bradley Cooper, would you catch the Zellwegger squints? If you happened across a sexual encounter with Keith Urban would your face lose all expression and texture for several years a la Nicole Kidman? Even more scarily if you slept with Jesse James you could either end up a trashy fame whore with tattoos all over your face! It’s a sobering thought.