Charlie and Ahlungor,
I agree with your thoughts on seeing a WL/ML with another punter. I reckon this has to do with the punting experience is you are renting the girl to 'be your girlfriend' for a short time. Anything that destroys this illusion sucks. Can be anything from seeing another punter, hearing her laugh with him at the end of a previous session, the girl hurrying you up because another punter is waiting. The list goes on.
I also reckon a working girl should never admit to having a boyfriend- bad for business
. Funnily enough I have found that if a ML admits she has a bf or husband she's not trying to put you off. It's more likely she is comfortable around you. Just my 2c.