What would you like understand about why a friend would sleep with your man or ex-man?

this morning, while getting ready for work, my husband was watching an episode of a sitcom called One On One.  i have never watched the show myself, but became intrigued with a heated discussion (no - argument is a better word) between two female characters. the two ladies were roommates and on this particular evening, one roommate was home, unexpectedly, when the other roommate arrived home with a guest - the guest, a guy her roommate had just stopped dating. she was not expecting to have to explain why she was creeping in with him, but before he could walk through the door behind her she quickly blurted out that she was sleeping with him. in walks the pink elephant of sisterhood.

in a span of a few minutes, both women resorted to name-calling and blaming the other for their wrong-doings when it came to men. the one who was sleeping with the ex announced to him that her roommate had actually been seeing her previous ex behind his back, as if that got her off the hook. finally, she broke down and admitted that she envied her girlfriend's life, was lonely and didn't like spending her nights by herself while her friend not only had a new guy in her life but an ex that still cared deeply for her and wanted to work things out. 

suddenly the man in the room became irrelevant. it was now all about loneliness, envy and genuine friendship. now, the entire argument escalated from slinging the "B" word around and calling each other skanks to forgiveness & understanding in 2.5 minutes flat. probably not a very realistic timeframe to comprehend why someone you call a friend, that you share your innermost thoughts & secrets with, would sleep with someone you dated, present or past. how could you comprehend your friend's choice of companions knowing that she knows it would be hurtful to you? would you ever trust her again?  would the friendship be over?

i don't believe that one girl friend's act of forgiveness could cure those feelings of loneliness or envy so easily. it would take a lot of soul searching and i still don't believe you could ever really justify your actions. ask yourself, how can one woman's personal inner turmoil explain hurting someone you care about, breaking the code of true sisterhood? if you caught your girl friend's man cheating on her wouldn't you want to protect from harm? how would you do that if the one doing the harm is you?

does anyone else besides me need help understanding breaking this unwritten code of "sister" honor?