It´s important to recognise where they´re coming from: From addiction, from healing process or maybe from the insights of beautiful energy of love.
In most of the case they´re coming from addiction to the peak orgasms ( clitoral orgasms or ejaculatory orgasms ). Those fantasies are going in to direction of perversion in order to bring harder stimulation required by deeper addiction.
Fantasies may come during or after tantric treatment, workshop, massage, practise or making love. Usually they´re sudden, not understandable and often there is no clue where they´re coming from. They may surprise us especially if we don´t remember, if we don´t recognise them from our life. Some past memories, some past conditioning needs to be broken, some liberation is manifesting in us. The healing is happening. The best in that moments is to just observe our emotions, energies, sensations, breath, body etc. and by being here and now, going through it (again ? ) in save environment, we would heal what it needs to be healed, we would liberate our self from something. Once and forever, we would get rid of , what is ¨ on our shoulders¨ and slowing us down and making our whole life heavy and miserable. Of course sometimes past memories, pain, sadness, other emotions etc. are to big to go through them at once. Be gentle with yourself, partner or client Sometimes to complete the healing you need more than one session. Don´t force yourself or others to more than you can bare. Its about healing not another traumatising :)
Other fantasies may come from universe, from beautiful energy of love as insights for us. The ideas how we could progress our sexual life, how we could bring never ending healing and love to even higher levels. This is only happening, when we already are bringing sexual energy to higher chakras, so we are not loosing it by the peak orgasms. In that way through fantasies, through third eye chakras, we are getting the insights, how we may use sexual energy in more loving, in more efficient and healing way for us, relationship and even whole universe. In those fantasies you¨re really feeling sure sacredness and even such a practise like BDSM could be used in order to heal the fear and trust.