Should you fantasize about having a relationship with someone you know who has expressed they are not interested in you?

Don't do that to yourself. You need to move on.

As a pure act, I can't see that it causes any harm.

However, it will very likely lead to frustration, anger, and poor treatment of the person in question. Those things are bad.

No, once you know it's not going to happen you need to do what you can to let the idea go.

Holding into it is just tying an anchor to your happiness that will only get heavier.

No, it will just hurt you in the end.

"Should you" as in "is it morally/ethically wrong?" No, it's not. "Should you" as in "is it a good idea?" No, it's a terrible idea. And I think you know that on some level.

In what reality would that be a healthy thing to do?

No. I know it hurts, but you need to let go.

As always, the dose makes the poison. An occasional fantasy won’t harm you, but an obsession is unhealthy and will make your life hell.

No, nothing helpful will come from nurturing that fantasy.

A relevant proverb I heard somewhere on a similar topic: You can't prevent a bird from flying over year head. But you absolutely can prevent it from building a nest in your hair.

as everyone here has stated, gotta let it go. the good news is: 1 down, billions more to go! just have to get out there and meet new people that will respect the amount of effort you put into caring about them 🙂

good luck!

I don't believe in the idea of "thought crimes" so morally you haven't done anything wrong. But its probably unhealthy to do that.

You're allowed to THINK whatever the hell you want.

The point is in keeping it to yourself an act civil.

No, it is not good. For the one who fantasizes that person (frustration, anger, jealousy, sadness) and then the other person (fear, mistrust). There is more than 8 billions people on earth. So, countless possibilities for connection, friendship, and love 😀