I’ve a relationship with a girl but also sex with my best friend. Is it bad to be bisexual?

There is nothing wrong with being bisexual. But having a sexual relationship with your best friend without your girlfriend knowing is another matter. If she finds out it may be very painful for her. Most girls want to be the one and only. I think it’s only fair if you tell her about your sexual preference. She must decide whether or not she can live with the fact that you also have sexual contact with your friend.