A reminder to engage in safe sex. Usually used when the people in questionaren't actually planning on having sex, but the speaker is implying that they will be having sex.
Person A is leaving the party with person B, a platonic friend.
While our lifespan is the number of years we will live, our healthspan is the number of healthy years we will have and our sexspan is is the number of years we will continue to be healthy enough to continue having an active sex life.