Board Thread:Show-Related Questions/@comment-84.181.185.61-20150824154643/@comment-26247132-20150824195837

STDs would only survive on the Ark if the original astronauts (Ark Grounders) brought it up into space with them (statistically not likely) and then shared it over the next 97 years. Even if they did bring STDs into space, with such a small initial population of 400, it's a lot easier to control the spread of disease and let it die out.

Besides, there's always the chance that most STDs have been cured and/or eradicated by the end of the world in 2052 (hey, they can make fully functional AIs by then), making sex less taboo (as we've seen) and less risky (as we've also seen).