To tell you the truth, this is probably one of the most significant threads we've hosted in a while. I know it's mind-boggling. And if we can make some progress through this, perhaps it will offer some real insight into how our cultural environment micromanages us - down to things that we consider "natural human responses".
This question is easily answered by doing a Google search for "nudism faq erection" .
Here's one of them:
"4.3. Gender-specific embarrassment
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 etc. during these times."
If you take the time to read the rest of the page:
http://www.toledotanclub.com/FAQ.htm
and then do some research on the AANR (American Association for Nude Recreation) and The Naturist Society, you'll find many intriguing options for living more natural lives and for the possibility that sexuality can be healthy rather than prurient.
Perhaps this topic can give us some deeper understand that many of the things we consider natural are really just culturally induced.