Two adults can agree to engage in sexual activity. Taking money for sexual activity is prostitution and if you take money to perform a service for which you are licensed and also, as an add in, perform a sexual service, you will lose your license and get charged with a felony.
I bet some clients would choose my legal services over my colleague's if I would offer a sexual hand release. Does that sound like a good idea to you? Does it sound legal?
This is at times referred to as a happy ending. However, it is a form of prostitution and if you were to be caught, you are subject to arrest, being charged and the ending is anything but happy for you. And yes, law enforcement can and do routinely make such arrests.
It is an illegal act, irrespective of the gender of your client, so a female undercover could cause you to be arrested just as easily if you are accepting female massage appointments..