No, it is completely inappropriate. Such a story involves vulgar and inappropriate sexual content that goes against moral and ethical norms in most social settings.
No. Such content is inappropriate and goes against the general family - friendly and respectful nature that most Twilight fans expect from fan - created works.
Yes, it is clearly an inappropriate interest. Such a topic is private and should not be a subject of interest among people in this relationship. It violates the normal boundaries of friendship and marriage.
In most cases, it can be okay as long as it serves a purpose in the story. Maybe the character is frustrated or exasperated and this action is a way to show their emotion. But if it's just there without any real significance, it might be better to leave it out.
Well, it depends. If it fits the character or scene and helps convey the intended emotion or action, then it could work. But make sure it doesn't seem out of place or overly odd for the style of your novel.
It could be that a group of friends went on a camping trip near a farm. One morning, they woke up to the sound of a very loud crowing. They went to investigate and found a huge rooster. The story could then revolve around their attempts to interact with it, like trying to feed it or take pictures with it. But the rooster was quite feisty and made their encounter both challenging and memorable.
No. Spying is generally not ethical. In relationships, trust is fundamental. If you suspect something wrong between your wife and your friend, it's better to communicate openly rather than spying. For example, in most healthy relationships, talking it out clears misunderstandings.