I wish you had mentioned what some of these fantasies were, for context. Most fetishes are harmless, until you depend on them exclusively for sexual fulfillment. Take the example of the (very common) foot fetish. By itself, it's harmless, it just means that feet would be very erotic to you, and can easily be incorporated into your sexual activities. I was with a foot fetishist for a long time, and for me it felt amazing to have my feet stroked, licked, rubbed, etc. while my partner worshipped them.

Fetishes only become harmful when it becomes the only thing you can do to get off. If my partner had stuck to worshipping my feet to the exclusion of any other sexual activity, I would have been very unsatisfied indeed!

There are some fetishes that can be harmful by their very nature. Necrophilia, for example, because you can't get consent from the dead. Bestiality and pedophilia are harmful by the same argument.

If your fantasies are kinky in nature, involve pain or humiliation or control and power imbalances between you and your partner, perhaps you should research into BDSM practices. BDSM is a safer way to explore darker fantasies in a consensual way, that provides boundaries and safeties to ensure that no harm is being done. BDSM can be incorporated into sexual activity, or explored without sex. Just do your research carefully, with a focus on how to keep it safe. And always use a safeword so you or your partner needs to communicate that things are getting unsafe.