Carcharians have superb immune systems, so he rarely gets sick. Even if he were to say... pay three dollars for a threesome in a shady space-cantina, the likeliness of him getting sick from it is relatively low. Carcharians are notorious carriers of STDs, though.

Virgil himself is clean. His ship's doctor tested him all the time JUST TO BE SAFE. (And as an excuse to jab him repeatedly with needles for being a douche bag.) Despite sexually harassing everything that moved back home, he was faithful to the two partners he had.