Politely refuse

"Sorry, Julie, but I don't think that's a very good idea," you reply.
"I don't have a temperature, trust me."

"Pleeaassee, John? I'll tell you what - if you let me play doctor, I'll let you play doctor. What do you say?"

You imagine yourself instructing this pretty young woman to remove her clothing, and think about gently feeling her warm breasts, and fingering her warm, wet folds, and all of a sudden you're reconsidering having your temperature taken.