A little while ago I wrote about what is normal and came to the conclusion that normal is not really something that is too worthwhile to worry about. It exists as a sort of imaginary mean between certain behaviors, but those behaviors at the extremes are not necessarily abnormal and given the broad spectrum of human activity and experience, it is difficult for something to truly not be normal.
That said, I believe there is a much more important thing than being normal to consider about our actions and ways of life. It is important that whatever we do, it is functional. The first consideration for something being functional is the simple question of whether or not it is even possible. Obviously if something is impossible to perform, then while wanting to perform it might not be too odd, an obsession too great with it can become an issue. Being too focused on something that is not possible can cause distress to oneself, but so long as ones fascination is kept within a healthy level that does not cause distress then it is fine. The same goes for interests that are possible for the most part.
Even so, there is also the problem of potentially causing distress to somebody else. This depends on how the interest is expressed. If the interest or fantasy or whatever else it is that a person may be enjoying is kept solely within the presence of their own mind, then there is no reason for anybody else to possibly be distressed by it. Of course this may leak a little bit into the realm of others for instance if you tell them that you have a certain interest that they find to be too abnormal or distressing for other reasons such as moral convictions, this tends to happen more with the possible than impossible kinds of interests. An example could be that you tell a vegan friend that you are interested in hunting, but have never actually hunted before and have no plans to do so. Sure, they might think a bit less of you for their own reasons, but if they were to do anything beyond that like stop talking to you simply because you expressed that interest, then they are clearly overreacting. However, they are not overreacting if you often express this interest. If you try to talk about hunting with them every time you see them, or constantly wear clothing with meat logos or an Elmer Fudd hunter hat, it can be pretty clear how that can make them uncomfortable and why they would not wish to associate with such a person.
It is when an interest is expressed that it can become dysfunctional, but no individual interest is in itself abnormal. Interests that are kept private and do not directly effect any other person though tend to be fine and while people who are close to you may have legitimate concerns about it, it is still ultimately your interest and if it is a part of you, then they should probably just accept it and not pry, avoid it as they can. Publicly expressed interests are another matter. As a general rule, if something you are doing is making other people feel uncomfortable and requires them to change something about their current way of life, then it is probably something best to keep to yourself. On the flip side, if not expressing an interest causes you distress, then there is a bit of a need for you to express it. Some people just love Harry Potter and need the world to know, dressing up as the characters, wearing whatever other merchandise they can find, and playing quidditch with their friends. This makes the matter much more complicated, but I still believe that there must be a recognition that the one who is performing the action is the one who is seeking to be accommodated and they should do so in as polite a manner as possible. This still leave question as to what happens when they cannot be easily accommodated, but perhaps that is best left for another time.