For brothers who have doubts in this area, take a read
Many sexually transmitted diseases may be transmitted via oral sex. Herpes is just as easily transmitted from the genitals of one partner to the mouth of the other as it is from genitals to genitals. Oral herpes (cold sores, fever blisters) may be transferred from mouth to genitals during oral sex. It is also possible to contract warts by performing oral sex on an infected and actively shedding person. Gonorrhea may also be passed orally, causing a nasty infection of the pharynx (back of the throat).
A barrier such as a condom on the penis, latex square over the vagina or a dental dam should be used during oral sex to avoid transmission of infections.
Other than the possibility of transmission of STDs, there is no problem with swallowing the semen. It cannot lead to pregnancy, and it is not very high in calories, either! For couples involved in a long-term, mutually monogamous relationship who do not have STDs, condom or dental dam use is not necessary either.