Please Help

hi..
i joined this site today because i dont know who to talk to about this.
the history: myself (20) and my girlfriend (18) have been dating for almost 2 years... we met at a week long christian retreat. she lived in another state, but this year, moved here for school so that we could be closer. things moved quickly, and as of january, we have been basicly living together.
the problem: one day in oct. i was looking thru the internet history (trying to find i site i had visited) on her computer and found many disturbing sites; gay couple chat sites, bi sexual dating chat sites, and an alternate email address she uses containing the word lebian.
i decided to confront her, i asked "is there anything really important your not telling me" and she said no.. so i went on and said, "it's ok, i know that your interested in girls, we can work thru this"
well, she got soooo mad that i would make this accusation, she insisted that she went to these sites because she thought it was really funny.. she promised me, and so im left with no choice but to believe her.

everything was fine, but recently i have had suspition that she was doing things again.. everytime i would come home from school, she wouldnt even look at me, i could tell that she was hurrying to close the internet before i could see what was on her screen, and then she would act all lovey dovey.

i know i shouldnt have done this, but it was really bothering me and i wanted to help her if she was infact attracted to the same sex, so i installed a program on her computer that would track the websites she visits.
i felt horrible doing this... but the first time i checked it, my suspitions were right. i checked back every now and then (i want to make sure before i approach her again about the subject), and everytime i had been at school, she had been visiting these sites.
i managed to see some of what she had been saying on these sites. a lot of it was very dirty and directed to the same sex.
in general, she was saying that she loves her boyfriend, but that she is bisexual and wants more then anything to expirience another girl.
i know she loves me, and i know she would never leave me, but one of these girls she talks to is 25, living in the same city, and intersted and getting together with her (as is she).
i dont know what to do. im completely stuck. i just found out that last stuff i told you, and im shaking. im scared ill lose her, im scared she might do something stupid. the list is endless.
i want to tell her i know and that its okay, we will work thru this together, but its so hard.
i want to tell her that this is wrong, but im affraid that she will then question this for the rest of her life, and continue on behind my back. then again, i want her to be happy (as i know this is causing her a lot of confussion and frustration too) and so ive thought about telling her she can try... but then im affraid she may like it and do it often behind my back.
im so lost. im praying but im to frustrated to hear the answer... please give me the best and most honest opinions you can.
i dont want to lose her, i love her with all my heart, and i know she feels the same about me.

thanks for your time, and im sorry i had to bring this problem here..

thanks for the quick responses everyone, i was really suprized.

the webpage idea was a good thought, but trust me, if she did get the refference, she would only take it as an insult and nothing good would come of it.
as per relationship counciling, theres no way she would even let me finish the sentence! she feels very strongly about keeping things to ourselves, hence why i feel so bad posting this.
i will have to talk to her.. you guys made some interesting points... but how does one bring something like that up? like i said, the last time i did, she got so mad at me that she locked me out of the room and wouldnt talk for about 4 hours, because i "didnt trust" her. and like Grishnak said, HE GOT STABBED WITH A PEN!!! i love my girlfriend, but when she gets angry, she gets angry!! so how would i avoid this??