This article is from the Naturism FAQ, by Dennis Kirkpatrick firstname.lastname@example.org with numerous contributions by others.
Naturists know that both erections and menstruation are normal, so
don't be afraid that your body's natural occurrences will offend other
naturists. An erection is embarrassing, but rare, and easy to hide -
just grab a towel, roll over, or jump in the water. Most naturists
won't even comment about it. (Don't fear the erection your first time
- most men will tell you they were too nervous for it to even be
possible!) Women may feel uncomfortable being nude during
menstruation. No one will be offended if women choose to wear shorts,
tampons, etc. during these times.