You may fantasize about sex with a family member, with an animal, with somebody who is much older or younger or about being overpowered or forcing someone to have sex. That does not imply you would like to experience these things in real life.

How to handle unusual fantasies?
1.      Know that it’s quite normal to fantasize about things that you would object to in real life.
2.      Realize that fantasy and real life are two separate worlds. If you talk about your sexual fantasies with a lover make sure that the other is able to make this distinction. If your lover interprets fantasy as something you long for, your relationship may suffer. In that case it’s better to keep them to yourself.
3.      Don’t try to get rid of your fantasies. They may easily become obsessive if you don’t accept them as a normal part of your sexuality. If you repress them it may also become hard to become excited or climax. As long as you don’t act on them they are harmless, like (day)dreams.