To me, if I was dating an SP, it's not a question of honesty, it a question of doubts.
I agree with normisanas in that it's important to know a persons reasons for being an SP. If it is a means to an end, yes dating material. Even girlfriend potential. I would expect a rough timeline or a goal to be reached. A career SP, it would take a much stronger and confident man than me to even contemplate it.
The time when she's still working towards that goal would be the hardest time. The trust that builds during that time would either be so much stronger or fragile than a normal civie relationship. Time after time in the threads on perb, you read about feelings being developed between client and sp. Sometimes one sided, sometimes not. Given the nature of the business it cannot be unexpected; this is a business that touts the Girlfriend Experience. Just because a person is trusting, does not mean they have to be naive. Given the types of men who hobby, I wouldn't be much competition. That nagging doubt would always exist. This speaks more to my underlying issues of confidence, but it would always seem like a question of time till she met someone "better" than me. I think it would be worse if we had met as a client and provider, because it would be already established that clients could turn into something more.
For the ladies, if you had a threesome situation where you both decided to invite another woman into your bed, then realized he was spending more time with the other woman, even though you know there was in theory no emotional attachment, completely temporary, and just a financial arrangement. It's the same type of feeling I would get every time she kisses me before she goes to work.
Another problem I think for me would be dating a "good" SP, especially one with excellent and ongoing reviews. Reading the reviews of her and recognizing the phrases and techniques and wondering if the intimate moments we've spent together were genuine or just part of her professional operands. Am I doing anything for her, or is she just making sure I feel like I am like her clients? Also as Yoko pointed out, after a long day of working as an SP sexual intercourse with your significant other can become a chore. Do I want to my needs to become a chore that she has to sigh, or grit her teeth through?