Sexual fantasies can take on two types of effects.
A fantasy or sexual fantasy may be a stimulation deriving from a positive or negative experience, or even both. 

It may be in response to a past experience and can influence future sexual behavior and the human abilities of cultural arts.

These effects can either posses a negative influence over the human brain (limbic system or dopamine) or this effect could also cause a positive influence towards the human psychological state of libido.

Sexual activities carried out by human beings on the Earth are transformed into sexual energies which possesses the power to influence (stimulate) the human passions of sexual desire.

Immoral behaviors and acts of pornography, homosexuality, sex magic, child molest, rape, and prostitution has the ability to produce negative environmental sexual energies, and also stimulate the nocturnal frequency. 

Due to the arousing pornographic images engrafted in movies, erotic art, children’s cartoon books, ancient arts, advertisments, subliminal messages etc, this has become the sexual fantasies which reflects the male sexual drive and psychological state of reward. 

Dreams of sexual fantasies (incubus and succubus–demons of masturbation), wet dreams, can become the result.

But on the other hand; the moral sexual activities which are practiced in a family (between a loving husband and wife), such activities has the ability to produce positive sexual energies or sexual sublimation.