Can Wishing Ring be applied on side-effects?
I think this is one of those situations that is pretty much up to the table to resolve.  See the interpretation in this thread:


  A Wishing Ring can remove a curse and its effects at anytime.
  
  So things like Big Feet, Tiny Hands and Chicken on your Head can be
  removed by a wishing ring even though after they have hit and are
  persistent.
  
  There is only one exception to this that I know of. Sex Change. The
  effects of the Sex Change curse can be canceled only at the time that
  the Curse is played. After it has taken effect the change is
  permanent. Playing a Wishing Ring afterwards can remove the -5 if you
  haven't been in combat but it can't revert you back to your original
  sex.


But contrast with this opinion:


  Wishing Rings have one function straight off the card, and that's to
  get rid of a curse. Whether it's to negate one that's incoming or one
  that's been lingering for a while, the Wishing Ring will undo any
  hated curse that would put your character in a detrimental situation.
  (Whether or not this applies to a curse that causes a sex change is up
  to the table in general, so long as it's after the fact.)


Munchkin isn't the kind of game where the interaction between different cards is always cut-and-dried.  I personally think it would be a bit mean to rule against a Wishing Ring being able to negate a Sex Change's -5 penalty, but it really does depend what kind of vindictive maniacs you have in your group!