although I am not a female, I would imagine that, unless the atmosphere is quite tense, then if just feels out of place.. I mean, you disappear out of sight and now she just has to lie there and do nothing? maybe she feels alone? or maybe she feels guilty for not giving anything in return?
although the alone feeling quickly disappears if you are doing you "job" 
also, as mentioned above, the feeling of not feeling clean enough, though of bad odours, bad taste, etc. might also be influential.. go shower together if this is a problem.. there is no cleaner place than in the shower
although it might not be the most comfortable place, nor the most spacious place.. actually it tends to feel a bit awkward... but in a sexy way...sometimes...
moving on to my personal life: there are some things that my girlfriend does not want me to do to her, because she feels very guilty of just enjoying and not being able to give back.. I do my best to tell her that I only do things because I really want to - in some cases it helps, in others more pursuasion is needed...
Feelings are powerful things - they override our minds and our bodies, as if trying to control us. We are but mere vessels for them to carry out their will.
Your Friend - Bjarke T.