Often, these stories can end badly. There could be jealousy issues that arise. For example, the bi guy might start to feel left out if his wife and best friend become too close without him. Or the wife could develop stronger feelings for the best friend than she intended, which could lead to the breakdown of the relationship between the bi guy and his wife or between the bi guy and his best friend.
There was a bi guy who had a wife. His best friend was always around, and they were all into some alternative relationship styles. The bi guy, after much communication with his wife, introduced his best friend into their relationship in a more intimate way. They shared hobbies, evenings together, and gradually, the relationship evolved into something more complex. The three of them learned to communicate openly about their boundaries and feelings, which made the situation work for them.
There could be jealousy issues. The bi guy might feel jealous if his wife and best friend become too close without including him. Also, the wife could end up feeling used or confused about her place in this complex relationship.
Sharing one's partner in such a way is not a common or morally acceptable practice in most cultures. However, in fictional stories, perhaps it could involve a situation where the bi guy has an open relationship with his wife and she forms a strong bond with his best friend over time, leading to a more complex relationship dynamic.
Often, they end badly. Either the friendship is completely ruined because of the betrayal. Or, if there was a romantic aspect, the relationship doesn't work out and they end up losing both the friendship and the romantic connection.
Often, they end with some form of resolution. Maybe the wife realizes her mistakes and apologizes, and the relationship between the husband, his friend, and the wife gets back to normal.
Well, sometimes the wife realizes her mistake and tries to end the affair before the husband finds out. But there are always consequences. The relationship between the wife and the husband is strained. The best friend may feel guilty and disappear from their lives. In other cases, if the husband does find out, there could be a big fight, and the end result is a divorce. The social circle also gets affected as people take sides.
Often, they end happily. The couple overcomes the initial obstacle of the brother's reaction. For example, in many stories, the brother is initially angry or shocked but then comes around and accepts the relationship. So, the two lovers end up together, happily in love.
Often, they end in divorce. When a wife's cheating is discovered, it breaks the trust in the marriage, and many couples cannot recover from that.
Some end happily. The two friends realize their mutual feelings and start a romantic relationship. They build on the strong foundation of their friendship and often have a very deep and loving connection.
Often, they end happily. Since they started as best friends, they already have a strong bond, and in the end, they usually live happily ever after, perhaps with a growing family or achieving their dreams together.