Here are some ideas I came up with for magic systems meant to lend themselves towards erotica. I like the first one the most. The other two are ok.
Body marking
All spells take the form of physical markings made on the body. They can take the form of tattoos, body writing, scars and birthmarks. To undo a spell, it would have to be covered by another spell or erased from the body. Partially covering or erasing a spell is a way you could modify a pre-existing spell. The size of the mark determines the strength of the spell.
Those spells could provide the individual they are with special abilities, such as the ability to fly, a means of modifying their body, as someone could change their sex and appearance nearly at will if they have the sufficient means and knowledge, but permanent and semi-permanent marks can also be used as a sort of magical contract or as a way of magically dominating another.
In this world, sentient and intelligent property might not be unheard of, due to how difficult it would be to even prohibit magical enslavement. Some societies might still try to suppress those kinds of spells or maybe magic all together; Others might embrace it as a way of controlling those who could otherwise not be controlled. Enslaving such barbaric beings as vampires so that they could not harm others.
Whispers
Some individuals are born with a special ability to influence others at close range and, in even rarer cases, the ability to influence them at very long ranges. Their abilities can include the ability to change someone's internal beliefs in radical ways, the ability to alter someone's will and the ability to make them "remember" and forget events.
People with such abilities would naturally flow to the upper crast of society, given how their abilities lend themselves to politics. Warfare might be fought with guns and swards but maybe even more importantly by whispers shared by the aristocracy.
Ghaist
Out-of-body experiences are a very real thing and are less sacred than usually thought of as. People's souls have their own unique shapes, representing their truest self to a metaphysical world. When one's body is acting fully on autopilot, some people's souls leave their bodies, but they aren't powerless. They can, of course, sense the material world, but depending on their shape, they can also influence it. Some might be able to arouse different sensations in others, others might be able to possess them, but there are also those who use this metaphysical realm as a way off hooking up with people without consequences.