Hi Ama.
Its a tricky question as i suppose its all down to the way you look at your job and why you are doing it? ie is it just for the money or are you actually trying to get pleasure for yourself as well?
In my opinion the sex you have with a client is totally removed from what you would experience in your personal like. You are an actress trying to create a perfect,sexy scenario for your client so that he will think he is as good in bed as he would like to think he is. This usually means lots of sexy moaning and groaning on your part. Whether you are actually enjoying it is totally beside the point,its what the client is paying to hear at the end of the day. Don't worry about them working out that your faking,99.9% will never cotton on to the fact or indeed care to be truthful,its all about the fantasy.
Which is int to say you should put up with a client doing things you don't like or that make you uncomfortable. But if they aren't then it might be good to start practising your fake oohs and ahhhs. You really don't want clients asking if they are doing something wrong,they are coming to you to have their egos fed and feeding their egos successfully is what turns a one off client into a regular.
Very depressingly shallow but true.