How do you know if your boyfriend has a bisexual relationship with his best friend?

I have a boyfriend who is bi w/ his freind when i first found out i was very upset some signs were he never kissed me around his "freind" and he would jokingly flirt but he liked it when his freind touched him its sick but atleast he isn't with another girl right?

To the answer above: How can you tell anyone a certain lifestyle is wrong or sick?