Kenw,
I do exactly the same, not everyday but fairly often, when I do not fell well or just for fun.;
Besides, I have been lucky enough to eventually find a girlfriend who shares my interest for rectal temperature.So sometimes if she feels horny, she will come in the bathroom when I shave, and,, surprise, surprise, part my buttocks and slide a thermometer in my rectum. If she is in the mood for it, she will grab my penis or balls from behind and take care of them. In this case, we usually end up back in bed and have sex. i will then keep the thermometer in my ass and my gf will play with it as I make love to her.
After sex, The readings of the thermometer have allways proven to be so to say dissapointing, sex does not make your temperature rise very much. If you want to rise your temp, go jogging....
When we indulge in this kind of play, we of course use plastic digital thermometers, to avoid accidents, even though we prefer the good old glass ones. LOL; .