I might get slammed for this. I think sometimes we as people in general turn to other people for alot of reasons. being an online thing to me is not as bad as meeting them in person. I know I have met people online that I have confided in and had "feelings" for them. to me it is sometimes we get in a vulnerable state. most of the time though I think people wake up before it goes too far. I would find it much easier to forgive something that happened online vs real life. I would also be asking myself what I was not doing for my partner that made her/him stray away.
I know there is still alot of pain in that situation but I would really try to work on the relationship together.