Sex is universal, so any spirit is capable to have sex with humans, as well as having a relationship with humans.
I would say that there’s three kinds of succubus/incubus entities:
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Pure Breeds - Creation of Lilith or her three Sisters, as well as creations of other high hierarchical authorities.
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Cambions/Nephilims - Union between humans and spirits, resulting in a mix breed with human and demonic/celestial DNA.
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Transformative Succubus/Incubus - Mixing and changing energies and DNA between humans and demons through complicated procedures that can last for days or weeks.
I have shared my own experiences HERE.
The latter is possible because of how “advanced” the human soul is. It’s transformable like clay, and changes through interaction with other spirits, adapting to the elements it exposes itself through.