Slim88, thank you for your post,
My idea, and what I achieved several times with a girlfriend a long time ago, was to have both of us a thermometer in our rectum(s) during love making. Of course,we used plastic electronic thermometer to avoid the risk of breaking a glass thermometer in the course of having sex. As I am not really a fan of sex toys, I felt kind of uncomfortable during the love making. My girlfriend was disappointed that the reading did not go up very much. Once, we tried it having anal sex and obviously, she could not have a thermometer in her rectum, and my readings there were still as low.
FYI, the reading for the first experiment we made were as follows ( in Celsius, I leave you with the joy of converting in Farenheit):
my gf: 37,2 before sex, 37,6 just after, 37,3 one hour later, me: 36,9 before, 37,5 just after, 37,0 one hour later.
To draw a comparison, after a lengthy tennis match in a hot Italian afternoon, my temperature reached 38,2 just after the match and back to 37,2 two hours later.