What do you think about me being in a relationship with my best friend's girlfriend?
Do you think this situation is tenable in the long run?
I'm sure my friend will get jealous of us after he hears about us.
Probably so. I think it's better to show some respect to your friend. He'd be really upset over this.
I'm going to send him an anonymous letter revealing what's going on.
You must not care very much for your "best friend." How would you feel if the roles were reversed? You might also want to reconsider being in a relationship with someone who would behave this way.