Well if she has any propensity to cheat he definitely increased the probability of that occurring. Also I'm guessing, The fact that she hasn't let it slide yet, will cause some inhibition on her half when you guys have sex.
My opinion, if she brings it up again why you did it. Just tell her honestly people have fantasies, that are strictly fantasies that they wouldn't want to have happen in real life. I'm certain she's had fantasies about being ****** to have sex or about being raped, when obviously in real life she wouldn't want that to actually happened. So just tell her honestly you were just wondering what it felt like wearing it, and you did it on impulse.
My opinion, if she brings it up again why you did it. Just tell her honestly people have fantasies, that are strictly fantasies that they wouldn't want to have happen in real life. I'm certain she's had fantasies about being ****** to have sex or about being raped, when obviously in real life she wouldn't want that to actually happened. So just tell her honestly you were just wondering what it felt like wearing it, and you did it on impulse.