There is a chance of getting STDs via oral sex: http://sexuality.about.com/od/saferse1/a/safer_oral_sex.htm

Here is a more balanced view
http://www.medhelp.org/posts/STDs/ORAL-SEX--STDS---confused/show/1507047

I am extremely clean and checked.  Hit me up if a juicy MILF would satisfy your curiosity.

Like they said, there is a possibility, but they actually make protection for that sort of thing as well, and I have seen gentleman who have brought them with them for their own safety. It's called a dental dam: http://www.nakedtruth.idaho.gov/dental-dams.asp