This is precisely why truth in advertising is vital in any industry. When expectations do not match reality, disappointment can have an over all negative effect on any encounter.
I have not had this happen to me, HOWEVER - if someone were to find themselves unattracted to me for ANY reason, I would hope that they be honest and upfront about it, and do us both the favor of leaving prior to any hanky-panky.
I would hope that they would offer me the same opportunity, as well.
Chemistry is terribly important in forging a passionate connection. Attraction can be influenced by any number of things; it is my belief that the best experiences are imbued with both. Anything less, and the parties involved are just going through the motions.
There is, of course, a right and a wrong way to extricate yourself from a less than desirable encounter; I prefer the honeyed approach (rather than vinegar). A simple, "I'm sorry, I hate to waste your time, but this will not work for me," should do, without flinging insults or vitriol. Alternatively, you could check your cell phone, feign concern, and say, "It looks like there's an emergency back at the office I have to take care of. I've got to go back right now; I'm sorry to have wasted your time."
It is up to the lady in question whether she may require a cancellation fee, and you may potentially have an issue on your hands negotiating your way out the door, but I truly feel that if you don't want to be there, then you shouldn't stay.