It depends on your personal values and the context. If the content is within the boundaries of what is considered appropriate and respectful, and if you are comfortable with it, then it could be okay. However, make sure it doesn't violate any ethical or moral standards.
There are different views on this. Some might think it's inappropriate as it can be seen as too much of an intrusion into the characters' relationships in a rather sexualized way. On the other hand, if the fanfiction is written in a tasteful and respectful manner, exploring the deeper connection between the characters, it might be more acceptable. But overall, it's a matter of personal judgment.