My personal opinion (and you may not like it): It's her life to do with as she wishes. If you guys are still friends (and want to remain that way), then let her know that you like/love/whatever her for who she is, whatever that may entail.
She's no different than she was 2 months ago, except for the fact that you reached in and ripped a personal secret (pardon the pun) out of the closet.
If you don't want to stay in a relationship with her (of any nature), then talk her into a 3some and bid her a good life when it is over.
Obviously you don't really trust her, since you felt it was OK to read her personal email and thus violate her privacy. So, looks pretty doubtful she's going to become a lifelong friend/partner anyway. Time to move on - you have a lot of life ahead of you, God willing.
