Sometimes in a love triangle the main person will make the other lover seem horribly bad to justify getting involved with the 3rd person. You're assuming everything she's told you is correct - what if it's not? Then only way this will work is if she completely breaks off with the other BF, otherwise you're both cheating. If she said she has NO intentions of breaking it off with the other guy - you should believe her.

You also have to think - would she do this to you one day and make you look like the undesirable one who deserves to be cheated on? You're participating in a love triangle. What does this say about her character and I think you deserve someone who only wants you. Keep your distance to avoid getting hurt. Imagine you were the other guy -there's no justification for cheating.