I've had a transvaginal ultrasound. It was not an easy or painless procedure. It was worse than a pap smear, in my experience. And my reaction to it was WITHOUT having had any kind of prior penetration-related trauma. I had not only consented to it, but I had been the one to request it, and it still was very unpleasant even though I had wanted it. I cannot even imagine forcing someone who was raped to have something like that.