This is how I brought it up to my, at the time, girlfriend.
Several years ago, on a Saturday afternoon, my girlfriend was over at my house. She said she felt like she had a fever. I had never brought up the subject of RT's before and thought now was my chance. I told her I could take her temperature but the only thermometer I had was a rectal one. She declined and no more was said.
Several weeks later we were out cruising around. We both worked 3rd shift and on our nights off we would just drive around. We were about an hour or so from returning home and I thought I would just get it out in the open. I told her I had a fever one time and had to use the rectal thermometer on myself. When I did I became quite aroused. Her only reaction was, "Hmmm. Interesting".
When we finally made it home she said she was going to take a shower. I told her I needed to, also. She told me to go ahead and go first. I did. When I got out I told her it was her turn. She gave me a sly little smile as she went into the bathroom. I wondered what that was all about. I soon found out. When I went into the bedroom, there, laying on the nightstand, was my thermometer and tube of KY. My heart started racing a mile a minute. She got finished and came into the bedroom. She asked, "Surprised?" I said, "Hell yes!" She told me that ever since I had mentioned it earlier she was curious about it. She told me to roll over, which I did in a flash. She then proceeded to take my temperature. This was a dream come true. To have a girl take my temperature. When she was done, she said something that nearly floored me. "OK. My turn." Another dream come true. To take a girls temperature. When I inserted the thermometer she said, "MMMMM. Kinky." From then on temperature taking became a part of out love making.
We married but eventually divorced. But from time to time we get together and 'play doctor'.