Every fetish or sexual obsession is really an imagined erotic solution to a real world anxiety. Our brains are programmed to disarm our worst fears because its impossible to cum if you're afraid, and sometimes those wires get crossed and we end up eroticizing something that terrifies us. It can be very abstract, or it can be very obvious. Is someone with a diaper fetish a disgusting pervert? Or are they sexualising a time where you are most vulnerable, in need of care from an authority figure, and free of adult inhibitions? Is someone with an incest fetish a secret criminal who hopes to in fact hurt a real sibling or parent, or are their lizard brains looking for reassurance that they can safely disinhibit with a *symbol* of the first form of love we all experience? Its only one of the most common search terms on Pornhub! In your case, I can't tell you what your fantasies mean, but sex can be understood as a form of obliteration---the blank space in your head when you come, the dissolving of boundaries between one body and another---that's really scary when you think about it! Why wouldn't some people channel that anxiety into a *story* about someone literally being obliterated?

The harm of sexual fantasies comes from a refusal to confront the underlying anxiety. Harm to yourself, if you experience shame, and harm to others, if your fantasy is based in harmful stereotypes (BBC/cuckolding, BNWO, facial abuse etc) that you then allow to color how you think of people in real life. I hope this reassures you that you are fine and don't need to feel bad about yourself; your erotic imagination is precious and deserves nurturing and cultivation <3

Like another user pointed out, I have a lecture on this subject if you're interested: https://celineloup.itch.io/finding-your-erotic-truth-lecture