Cheating is wrong in the way it demonstrates a lack of respect for the other person as a thinking, feeling, conscious being, and someone that you care about.
What it really means to cheat is to break a shared emotional understanding that you have with another person, undermining them as people.
The consequences for the other person, emotionally will compound the immorality, especially being their partner and best friend, you of all people should know better.
Its one of these topics that is very dependent on the individuals and the context. Factors such as mutual understanding of having multiple partners will obviously nullify the moral implications, and sometimes a break down of communication can lead to one person acting in a way that he/she thought was ok, but in fact hurt the other quite significantly.
I do think that most of what constitutes immoral behaviour stems from acting in a way that presumes things about another person, things you could not know. That undermining and lack of respect for them is a form of objectification or belittling, with the person unjustly assuming some inherent superiority to the other.
Alex