I have had a fetish for seeing a girl in a mask since I was just beginning to have sexual feelings (and too young to understand them as such). If I were to participate in a medfet exam, which I haven't before, I really have no opinion of what the "doctor/nurse" should wear. I know there are rules on that, and I'd want to see the rules followed. I like authenticity, I guess.
If it were the patient that was masked, which is something I DO have an opinion on, I'd prefer to have her in either a 3M 1860, (which is one of those N95-rated, turquoiseish blue masks that are thick and have two white rubber bands to hold it onto her face) or a 3M 6291 half mask respirator with pink disc-shaped filters (2091) on each side. It's not too common to see the half-mask respirators, but they are widely used for patients at Stanford (I feel like they're starting to use the 3M 7500 series now, with the same filters, but I don't know).