While they were historically viewed negatively, the current belief is that sexual fantasies are positive and indicate a high degree of sexual desire.
It is entirely normal to have thoughts of, or even enjoy acting out, the experience of being handcuffed or perhaps strapped to the bed during sex.
For example, a woman may fantasize about hooking up with another female while her male partner watches or she may desire to simply have a one-time sexual interaction with another woman.
Since this is illegal, it is best to keep a fantasy.