"If I go out, then she may feel justified in doing the same (now 'legalized', by me, for her also) thing, "
She already does feel justified to do it. She told you flat out she was.
"The 'helpful' magazine was laying opened to the page on internet dating (subtitle "here's how it works"), which I found distasteful, considering my d15 could also see it. "
Are you really sure this is what she did? Sounds childish and not something she would do.
M-43 W-40 2D - 9 and 5
Emotion, yet peace. Ignorance, yet knowledge. Passion, yet serenity. Chaos, yet harmony. Death, yet a new life.