Then about a month ago, I was on his computer and I found out why he was so secretive. He had been chatting and e-mailing with other women and also exchanging pictures--both clothed and nude. I confronted him, and he was horrified that I found out. He showed me the picture he had sent, which wasn't even of him, and he said that he never actually met any of these women.
But how can I trust him again? How do I forgive him? I have prayed about this, and talked with several Christian friends, but every day since this has happened, I can't seem to think about anything else. I have so much fear that it could happen again.
First of all, your sense of betrayal is absolutely justified. Marriage, or the commitment to be married, is about achieving intimacy at every level--physical and emotional. This means that couples should not hide anything from each other. Your fiancé transgressed this vow, first in his reluctance to share his passwords, then in his relations with other women, be they cyberwomen or otherwise.
I suggest therefore that you:
Practice trust by emptying the contents of your heart to your fiancé. Bring the relationship to a deeper level, and hopefully he will reciprocate. Then he won't need to discuss these issues with anyone online. He'll have you with whom to practice a radical honesty that should lead to a deep and lasting intimacy.