I've only observed one other lesbian on this board besides myself, so I'd be interested to read any responses.
As far as my advice on your question, I think a lot depends on whether you are the submissive partner or not. Do you want your girlfriend to take your rectal temp, or do you want to put a thermometer up into her? I would imagine the latter would be a lot harder to introduce as a brand new subject. Do you know if she is at all anal-erotic?
If you only want to receive, try to start a discussion with her about how you've learned that some people have secret fantasies, have fantasies that excite them in a way that many people would not understand. Gauge her response. Discuss different types of kinks, and see what she says. Gradually work your way around to anal eroticism and see what she says. Finally, confess your secret desire to be penetrated that way with a rectal thermometer. Even if she does not agree to satisfy your fantasy right away, all is not lost. She will now know the true you, understand you better, and perhaps the idea will ferment and come to fruition in the future.
If you want to take your gf's rectal temp, I'd casually mention that you'd accidently stumbled across some reading on the Internet that you found oddly exciting. Give her some erotica from the 'Net or even this site to read. After she does, even if she expresses distaste, ask her to submit to you for your sake. If she is vanilla, you have a chance, but if she is Dominant, you will likely never succeed.
Good luck, and I'd love to hear the details of the encounter.