The Rama says (OC 38:3) that one must protest a woman's putting on Tefillin. The Magen Avraham there (sk 3) says that this is...:
מפני שצריכין גוף נקי ונשים אינם זריזות להזהר...
...because [Tefillin] require a clean body, and women aren't quick to be careful [about this].
What kind of cleanliness are we talking about? It's kind of hard to believe that women (even 400 years ago) would need to go to the bathroom uncontrollably.