I've never been concerned about where the toilet stop was in the field, even if I had my period, I just accepted it was tough.
I've carried bags for sanitary products along with my field kit on many a field trip, and had horrific experiences of hiding behind the only bush in miles! However, a field trip to New Zealand with students helped me re-evaluate that position and made me question why I had accepted it was okay. Part of the reason was that suddenly I was in an unfamiliar environment and whilst there are no wild animals lurking that would do me harm I was less comfortable than I was in the UK, but the big change came because a female student make herself ill because she really didn't want to 'pee in the field'. Since that episode, I have ensured I always know the location of toilets close to and/or at field sties, or I schedule stops en-route to sites. I now ensure that all coaches used for university trips come with on-board toilets, where known sites do not have the facilities. This policy has been adopted by my colleagues and our field trip coordinator only books coaches with on-board facilities. We made students aware of this policy change, and are up front about conditions in the field when it comes to toilets. The idea that you can just pee behind a wall has been relegated once and for all.