Don't misunderstand me. I fully believe that a sexual relationship between a doctor and a patient is reprehensible and unprofessional, and often an abuse of power. I would say this particularly applies when the doctor is a gynaecologist.
However, the idea of an ultrasound while having sex is at least intriguing.
The best ultrasound I ever had was a penetrative internal one. I could see my ovaries and follicles on the screen looking really rather beautiful.
I would highly recommend it. Only to women, though.