Would really depend on the client.
Some would be just getting it out of their system, so it's hard to say that their wrong just for being aroused by something almost all other society says is taboo.
It's the other type, that aren't perhaps as self aware and capable of distinguishing between when the line has been crossed or not.
The unstable ones.
This could be a form of test run for them and ultimately gives them a sense of entitlement about their true desires.
Personally I truly don't care what 'consenting adults' get up to in the bedroom. If everyone is there because they want to be and everyone is participating in things they want to be doing.....
Well then, no matter how far out there for my tastes it may seem.....
More power to you people, have fun, whatever.
The second you cross that line though and either use the complex emotional bonds of family to manipulate a minor into something they can't possibly have any idea of the full emotional and physical ramifications of, well, you've crossed the line.
Same goes for manipulating and or forcing people into things against their will. You've crossed the line.
Some people may try and argue that the lines can get blurred, but in this context, I don't think so.
Consenting adults, pretty simple really.
Healthy role-playing for some, v's, creepy, you get the feeling this person shouldn't be going there for others.
If a client is giving you the creepy feeling, just say no, I'm sorry but I'm not comfortable with this type of role-playing. If you already did once before and they bring that up, just say 'yes and it left me feeling uncomfortable sorry'. Hopefully you might help them realize something.
Hopefully you might even stop them from thinking about crossing that line in their real life.
Good topic Alix, albeit a very complex and touchy one for a lot of people.