This is my understanding, and while I am not a doctor, I have worked several times in a health and sexual education bureau.

A condom is highly effective against all STDs save two: Genital Warts and Herpes. All that is required for those two diseases to be spread is skin contact between the infected area and an uninfected victim. With Herpes, abstinance during an outbreak is the only course of action that will not spread the disease. Genital Warts sufferers must either entirely cover the warts or refrain from sex - but Genital Warts, unlike Herpes, can be cured.

All STDs can be transferred by oral sex, but some are more likely than others. HIV will only transfer if there is a wound in the mouth, but that is unfortunately common. Virtually all of the common STDs thrive equally well in the mouth and Esophagus as in the genitalia. Oral dams are, unfortunately, largely useless, since the entire interior of the mouth is mucosal membrane, and thus an acceptable medium for these diseases' growth.

Ejaculation on the skin is less dangerous than inside the body, but is still subject to certain dangers. Any cut or lesion is an entry point for these diseases. And one of the most common STDs, Syphilis, is perfectly happy going through the skin instead of mucous membranes.

The best way to protect yourself is to have a single partner, and to get both of you checked out by a general practitioner, including a comprehensive blood test. What you don't have, you can't pass on.