That moment when you notice something that, in retrospect, you feel should have been totally obvious.
Granted, Tedd could have been born with a natural talent for gender-based magic. Of course, he was ALSO apparently born with some other natural talent that lets him see how spells work. Perhaps it's possible to have two natural talents in regard to magic?
Of course, there are several possible explanations for why Tedd was able to be given a mark, possibly including explanations that might not be included here.
There is, however, one particular explanation that, against all odds, is most definitely not the case: A wizard didn't do it.