Alternate explanation....Maybe she's looking at those sites in order to find ways to spice up your relationship?

Its important that you don't attack her. Have a discussion with her after you've calmed down a bit. Maybe she is just curious....at this point, maybe nothing has actually happened and she hasn't even met (or talked to) anyone else. Its possible that she had a moment (or two) of weakness and started looking at those sites out of curiosity.

Maybe she's feeling lonely, maybe she's not feeling loved, maybe... Ok, so I'm not excusing her behavior, but at this point you have no idea if its just internet browsing or if she has acted on anything.

I'm hoping that at worst she's just feeling a bit unloved or even bored and is exploring what else is out there. You don't even know if it was pure curiosity.

If you attack her, I GUARANTEE you that your relationship WILL go downhill. Either she is indeed looking elsewhere and will end up firing back at you, or she is just curious....and if she's just curious, then you've created a huge breach of trust between the two of you.....I would feel VERY violated if someone searched through my internet history and used it against me even if I was just bored and hadn't actually done anything wrong.