Otherwise she might assume that I had gone to prison for 30 years or something ridiculous like that

the male nurse might assume I'm gay otherwise

What you need from the nurse is her set of skills. Her personal opinion of you is irrelevant to doing her job. I understand that we may see health professionals as higher-status than us, but they're actually doing us a service. You don't need to feel intimidated by an unspoken imagined condemnation.