I posted this on another relationship forum, but I didn't get much advice out of it. I'm hoping here will be better!
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First off, I'll admit my age - I'm 21, and I've been with my fiance, who's 23, for over five years now. He's of course the ol' high school sweet heart, and granted we've had our problems, we never really broke up, we always managed to work through them.

We've been living together for close to two years now, there's no financial struggle and we were finally getting things ironed out for our wedding at the end of this year. I thought everything was fine, we haven't had any serious arguments, but today I was hit with the worst news in my life.

For the past month, he's been saying in appropriate things to my 14-year-old sister and her best friend over Facebook. He's been saying my sister is turning out to be "hot like her big sister." She played it off as just a compliment and didn't report it right away. Two days ago, her friend showed her a log she'd been keeping (it's only a week or so long) about how he's also complimented her, saying she looks "really hot" in certain outfits with lace, etc. She reminded him that he's 'seeing' someone, but he used the 'we've bee fighting a lot' card to keep talking to her. I guess she finally gets creeped out and shows my sister who then shows me AND our parents.

The extent of their messaging, from what I could read without breaking down in tears, was things like he "wishes to hold her" and "he'd date her in a heart beat, if the circumstances were differnet." I'm a little upset that my sister's friend didn't approach someone about this sooner since it's been going on for a few weeks, but being she's young, I can't blame her.

We confront him, he breaks down crying and admits that he may have a problem. He also says that he didn't think about consequences before hand nor the fact that she was 14. I keep trying to figure out if he has an attraction to young girls, which he's denied, or if he was honestly 'losing himself' over the computer. Never in a million years did I see this coming. Was I dating a pedophile for five years? I'm so hurt and confused that he'd scare my sister and her friend like this - what do I do? He said he'd get help, and even went to schedule an appointment with a therapist today, but will that even help him? I know I must choose family first in a relationship like this, yet that reality alone hurts me even more.

My question is this: Do I help him get counseling and try to reconcile if he's so sincere, or just dump him all together? My father has been the most understanding, where as my mother believes I should 'do what's right,' whatever that is. She believes in the 'once a cheater, always a cheater' thing. Ending our five year relationship would honestly kill me, but the betrayal has been haunting me for a good 48 hours now. My sister seems to be acting like everything is fine, and I haven't seen the friend he talked to.