I will add to the above that oral transmission of hep-c, hep-b and HIV through semen is considered to be possible.

There are a number of studies in conflict with one another as to the actual risk, but from what I last read (some time ago) there's at least some credible evidence that it is not as low-risk as was once thought.

However, given the unreliability of self-reporting and the ethical problems with asking someone to expose themselves to HIV or hep-c for the purposes of actually determining the risk, it it not likely we'll have a definitive answer to that question any time soon.