Perhaps its the idea of having oral sex in general that you're really put off by? There is of course nothing wrong with this - not everyone is into this kind of sex. I personally am not a huge fan but with sex in general, good hygiene is a must. Have a shower with your partner beforehand or maybe have a convo with them to state plainly that the idea of oral makes you nervous and that if you knew you were both clean beforehand it would make things easier. Sounds silly but sometimes to get what you want you've got to ask for it. Im sure no guy would have a girl go down on him or vice versa without being clean first anyway - it only adds to personal insecurity let alone discomfort for the other partner. All the best.