Sam2 – Guf Naki has nothing to do with hygiene. It has to do with being careful not to expel gas while wearing Tefillin. What’s more is that there is also a Chiyuv to have your mind on the Tefillin at all times and to be in a state of reverence. This the Mishna Brura states that women are less careful with than men. Therefore, your average woman should refrain from wearing tefillin.
Furthermore, if not for the fact that it is a mitzvah D’orisah, Chazal would have forbade even men from wearing tefillin due to the reverence needed. Originally tefillin was worn all day. Chazal limited it to while davening Shachris.
Being that it is not simple for the average Jewish man to wear tefillin, all the more so women who are not obligated to wear Tefillin.
Those men who entertain the idea of women wearing tefillin do not fully understand the reverence and kedushah of Tefillin. Better to start focusing on our obligation and relationship to wearing Tefillin instead of worrying whether women are allowed to.