Sexually associated diseases are ones that you don’t get from your partner during sex. Instead, sexual activity itself increases your risk. For example, some women are more prone to urinary tract infections (UTI) when they are sexually active. This is not because their partners infect them during sex, but because irritation of the urethra during sex, among other factors, increases their risk of UTI. Other diseases that are currently considered to be sexually associated, like bacterial vaginosis, may have a sexually transmitted component that has not yet been fully identified.
The classification of a disease may change over time as scientists improve their understanding of its transmission. For example, cervical cancer was not commonly recognized as a sexually transmitted disease until the 1990s. Bacterial vaginosis may be the next disease to move from the sexually associated to the sexually transmitted camp. Some scientists have hypothesized that there may be a sexually transmitted virus that infects the normal bacteria of a woman’s vagina (not the woman herself!), and allows the organisms associated with BV to overgrow.