A few things based on experience. Situation: if a needed procedure in a medical venue it could be a female.
Thermometer: in the past, the most common were glass mercury. They were replaced by electronic units or digital.
Familiarity: The female using whatever variety needed to know how to use the instrument.
The use whether by a medical or non medical female dictated how it was effected
Attitude/ culture/,experiences : Outside of medical venues I would say culture , attitudes and experiences say whether or not your RT will be taken.
I have had many taken rectally any number of times as an adult by females in the medical arena, both glass mercury and electronic units employed.
Non medical in role play and of late massages where I have had my RT taken. My preferred instrument is a glass thermometer. A stranger to many young women; and it showed. Some didn’t know which end to insert. One laid it between my cheeks. Reading them…I usually had to do that.
So there are a variety of factors. One young woman asked me to demonstrate it on her. I did. Like her, as she told me; as many the females also said they had orals done. Rectals were new to them.