Having someone on the side is disrespectful. Going for your friend’s boyfriend is bad karma. Choosing to go out of your way to prove something to yourself while knowingly hurting someone else has its consequences. Insecure people hurt others.
The bottom line is: respect others and their relationships. Whatever you put out into the universe returns to us all ten fold. Thus, if you know two people have an intimate relationship: respect it! Relationships like love are not something you chase, have faith that you will have friends and lovers who want to be with you. Be patient with the process of life and do your best to respect others. You get what you give!