if you keep a secret too long it gets harder to tell someone, i agree this site can be qoute graphic and im sure theres even stuff on here you dont like, but there are tamer parts, Now maybe a good time to show her, hopefully she will be understanding and respect your tastes (i mean it could be worse), it should remove all suspision and tension that could result. It may not be a good idea to just point at the screen and say "this is what i want you to do"
show her your interested in this and take your time, your not trying to make her, because if shes not enjoying it, you wont either. You have kind of allready missed a good opertunity when she saw you, maybe she could catch you again, if you get my drift, it had the oppertunity to be a good starting point, but be carefull it could get messy. Start with the tame light stuff (as not to put her off immeadiatly), with patience from the both of you it could turn into something beautifull.