Found this on the net...

HSV can also cause pharyngitis, an inflammation of the throat with similar ulcer formation. HSV is highly infectious and usually sexually transmitted. It is certainly transmitted by penetrative intercourse but there are several reports of transmission through oral sex. Virus particles are shed profusely from ulcers, either oral (including cold sores) or genital, and infection is far more likely when these are present. Transmission is unlikely, but not impossible, in the absence of ulcers.

If you have a cold sore on your lip, do not give anyone oral sex!