Well, I had assumed the narrator to be the spankee (as I stated when I posted to this poem), because of lines like "A swish in the air keeps me alert" and "OMG! It can't get much worse," which suggest the reactions of the person being switched.
Still, in reflection that's hardly conclusive, and there are other lines which could imply aome degree of detachment from the punishment--as though it could be the narrator wielding the switch but trying to stay emotionally uninvolved.
At the end, the last stanza starts out "Gently wiping away his tears, I finally speak without fears," which normally would suggest that the other person (a male) had been disciplined--unless the spanker had ended up being disturbed by having to administer the punishment was was crying sympathetically him/herself.
The last two words, 'trusted me" could mean "trusted me to discipline you as you deserved and/or desired" or "trusted me to accept the discipline you felt I needed or deserved."
Perhaps the poem is meant to be highly subjective, its title ("The Switch") could definitely refer to the reference point (narrator or other character) switching from stanza to stanza and in some cases even from line to line.
The only thing that I'm certain of now is that the F/F gender orientation isn't a possibility...

--C.K.