Only if it is has a sexually transmitted infection in it. Otherwise, taking semen into the mouth is not harmful. Swallowing is not harmful either. And for those who may worry about it: a woman can't get pregnant from swallowing semen.
However, keeping semen out of the mouth is one way to make oral sex safer. Unprotected oral sex (without using a condom or other barrier) puts both partners at risk for a number of sexually transmitted infections, whether they are giving or receiving genital stimulation. These infections include cytomegalovirus, gonorrhea, hepatitis B, herpes, human papilloma virus (HPV), syphilis, and, rarely, chlamydia and HIV.
For safer oral sex, use a condom to cover the penis, or a Glyde dam or plastic wrap to cover the vulva or anus.
