Its your money and your life. Do whatever you want. If she is willing to accept payment for her services, then its a business transaction. If she is having to do it because her pimp beats her down if she doesn't...well....then you
REALLY have to do it.
...Just trying to keep the girl from getting the backhand.
If she is a slave...then maybe you should reconsider and let the next guy in line do it instead...unless of course your into that sort of thing. If so, then go for it.