A few times in front of family members. One winter when I was 13, almost half the family was spending their vacation at my grandmother's. The kids, so my generation, shared two large bedrooms in this house, four beds apiece. One of the younger cousins landed himself with a flu, so the aunts and my mom regularly took our temperatures to make sure nobody else was coming down sick. It was a once-in-the-morning once-at-night thing, over the course of a few days, of course with an extra temp taking if any kid complained about chills or had a warm forehead.
Anyway, it wasn't the end of the world because everybody got the same treatment and we mostly hid beneath our blankets while the thermometers registered. One night however, my cousin Marianne and I (we were the two oldest, me at 13 and her at 12) stayed up much longer than the other kids, finally wanting to roll into bed sometime after midnight. Our moms, not wanting to wake the cousins already sound asleep, but still worried about the flu, decided to take our temperatures on the couch in the living room. Luckily for us, almost everyone had gone to bed or was elsewhere, because it was still pretty embarrassing for the two of us, draped across the L-shaped couch, facing each other with our jeans at our thighs. I couldn't help a bit of excitement of course, but Marianne and I still blushed brightly when our panties were lowered and each of us got a thermometer slipped into our bottom.
As younger kids, we'd been unclothed in front of each other before, but something about this was different. I was already at the age where I was just starting to enjoy this, so it definitely helped that Marianne was in plain view of me, a thermometer nestled in her butt, with her staring wide-eyed and pink in the face as I got my own temperature taken.
Of course, neither of us had a fever. Definitely for the best, though I'd have loved some suppositories.