I mentioned in the OP about the binding magic having a sexual component. Here's some more developed thoughts on how this whole binding thing works. I want to know what you all think before I carve it in stone in the OP.
All binding spells use a series of symbols and a large circle (similar to alchemy in Fullmetal Alchemist). The circle is often two circles, forming a ring, and the symbols are painted in the band, between the lines of the ring. There's actually two different languages of summoning magic: one summons only angels while the other only summons demons (and you guessed it, later on, new languages and supernatural creatures may also become a thing). The traditional symbols like crosses and pentagrams aren't used in this form of magic, but a central symbol always appears in the summoning circles.
Summoning an angel or a demon also requires some kind of physical object. For an angel, a feather dipped in holy water is used, while for demons, any gold objects dipped in blood. Once bound, this object acts as a sort of... phylactery. It becomes nearly indestructible, and also cannot simply be lost. If a summoner misplaces or drops it, or if it is stolen, then it will magically appear in their pocket, or in their room... somewhere where they will find it quickly. They can however give it away, trading the summon, or destroy it themselves (and only they or a very powerful being can do so), thus freeing the summon from their bind.
Once summoned, the angel or demon, unless it is of level 5, will assume a mortal body. Depending on their power level, this body can be just as frail as a human's. Once their body dies, they return to the celestial planes, and are freed from the binding. When summoned, they will appear in the summoning circle. Angels always appear from a flash of light coming from the center, and demons always appear from a fiery explosion (one that, magically, does not catch anything on fire, but can leave some ashes to clean up).
The way the magic works, the binding is not fully completed until a specific sexual or romantic action takes place. All angels work the same way, and all demons work the same way. Until the action is performed to completion, the summon will not be fully bound, and the more temporary binding will wear off naturally in about three hours. In other words, once summoned, a summoner will have three hours to fulfill this specific sexual action on their summon, or they will become fully free.
For a demon, the actions are as follows:
Level 1 - A plain kiss to the summon's genitals.
Level 2 - A single but thorough lick of the summon's pussy, or a single good suck down to the base of their cock.
Level 3 - Oral sex to the summon, and the summoner must consume some of their cum.
Level 4 - Vaginal or anal sex with the summon until they reach an orgasm. Don't forget that summons at this level can change their bodies to work with this.
Level 5 - Irrelevant. The sexual binding does not work at level 5.
For an angel, the actions are:
Level 1 - A simple kiss to the lips, tongue not necessary.
Level 2 - A make out session at least fifteen seconds long.
Level 3 - One way or another, the summoner must cause their summon to reach an orgasm.
Level 4 - Vaginal or anal sex until both the summoner and summon reach an orgasm. Don't forget that summons at this level can change their bodies to work with this.
Level 5 - Irrelevant. The sexual binding does not work at level 5.
After that, the mind-bending magic starts to work. The summon will, often slowly, develop feelings and desires for the summon. Some try to fight it, but all ultimately succumb if they stay bound long enough.
