Dear Another,

It's not an actual temple policy. It used to be a rule because when tampons were first around, young women didn't use them as much, and before then, there were no tampons. So it was for sanitation/cleanliness issues, but with the advent of tampons and menstrual cups it's not an issue anymore. It took a few decades for everyone to catch up to the new policy, but that's where things are now.

–Concealocanth

Dear Another,

Like Concealocanth said, it's not a Church-wide policy. I personally can't think of any logical reason why it would need to be. If it was for sanitation reasons, they'd make all the smelly teenagers shower before going in the font, and if it was about laundry, they wouldn't let menstruating sisters even wear the jumpsuits for confirmations. There's a long history of women being symbolically "unclean" while on their periods (especially in Law of Moses times), but nobody's barred from doing endowments while on their period. If it's symbolic of anything, it's probably only symbolic of having old-fashioned and kind of sexist ideas about how "icky" women's bodies are.

Although it's not an official, Church-wide policy, some individual temples do still hold to the old policy, whether that's because of the temple presidency's official decision or because of decisions of individual baptistry workers to do things that way. Recently, the bloggers at Feminist Mormon Housewives brought this issue up in a series of posts, and they mobilized their readers to call all the temples they could and ask what each temple's official policy regarding the matter was. You can see the data they're collecting in this spreadsheet. At the time of the last post about it, they reported that three-fourths of temples contacted either had no policy about it or allowed it, provided the sister in question was wearing a tampon, but one-fourth of temples still had an overt policy against menstruating sisters getting in the water at all. Sigh. Still a long way to go.

- Eirene