In most cases, it's not okay. Your best friend trusts you with her relationship, and engaging in such behavior violates that trust. It can lead to a breakdown in the friendship and a lot of emotional turmoil for everyone. It goes against the basic principles of loyalty and respect in a friendship.
Yes, it is morally wrong. Cheating in any relationship, especially in a sexual context, is a betrayal of trust. It can cause a great deal of pain and emotional damage to the partner who is being cheated on. In a sex story or in real life, it goes against the principles of loyalty and respect in a relationship.
It's not appropriate to share explicit sex stories casually. Such topics are very private and should be kept within a more appropriate and consensual context, like in a healthy sexual relationship education session led by professionals, not just in a normal daily sharing.
It's highly unethical. Consider the situation from your best friend's perspective. She has an emotional connection with her boyfriend, and by getting involved sexually with him, you are not only breaking her heart but also ruining the bond you have with her. This kind of behavior is not acceptable in any healthy relationship dynamic, whether it's friendship or romantic relationships.
No, it's never okay. Cheating is a betrayal of trust. In a relationship, your partner trusts you to be faithful. If you have issues in the relationship, it should be resolved through communication, not by seeking intimacy outside the relationship.
One of the main consequences is the loss of your best friend. No one would be okay with their friend having sex with their partner. Your friendship will likely be broken. Also, it can create a lot of drama in your social circle. People will take sides, and there will be a lot of gossip.
Sharing such stories is inappropriate as they involve unethical and private matters. We should focus on positive and healthy relationship topics instead.