She takes the initiative.

"You don't seem convinced" says Dr. Bensen standing up. "I assume I am about the age of the women you are attracted to? Or perhaps perhaps you like younger women?"

"Yes. I mean No. I mean you're exactly the right age" you stammer.

"Well then" she continues kicking off her shoes and then reaching up under her dress and pulling off a pair of white cotton panties "Given that I have a vagina that is about average size for a woman of my age and level of sexual activity, which is only slightly below average itself thanks to my busy schedule, I believe I can give you an accurate evaluation of how your penis will feel during intercourse."

She goes over to her desk opens the draw and pulls out a condom. "Now I'll need you to put this on and then engage in sexual intercourse with me in the manner as close to that which you would usually use as possible given our lack of the usual environment. I will then evaluate, more as a sexual partner than a doctor, how enjoyable the experience is. Do you trust me to be open and honest about this?"