Celestial Succubi

A note on gender: Traditionally, succubus is used for girl sex demons and incubus is used for boy sex demons. But Etymologically, "succ" and "inc" refer to being a bottom/top (with gendered implications, of course). Sex demons and angels are, of course, equally happy to bottom or to top. And I don't wanna write "succubi/incubi" each time. And "concubus" sounds dumb. So the official word is: there can be boy succubi and girl succubi. 

Most people I am likely to spend much time with don't think that extramarital sex is a sin. If that's the case, there's no reason why demons should get a monopoly on sexy temptation.

Angels

Angels are perfectly moral beings who will not sin under any circumstances (At least, they conform perfectly to a certain moral code, if you want to get all Hume's Guillotine about it). They come in a variety of shapes and sizes. They usually, but not always, have cool magic. They usually, but not always, are very clever. It is unclear how many angels there are, or why they intervene in some evils but not others. Their ability to get anything done in the mortal world is somewhat limited, because they never sin. This means no violence, except against demons (and maybe undead?).

Succubi are a kind of angel that is specialized in having sex.

Sexual Morality

There are all sorts of reasons why a succubus might have sex with a person. If you're feeling sad and alone and misunderstood, a succubus might show up to help. If you're an evil tyrant or warmonger, a succubus might gain your trust and try to tempt you into committing good deeds. A huge quantity of smut has probably been inspired by the antics of various succubi.

Of course, just because not all sex is sinful doesn't mean none of it is.

• A celestial succubus will never have sex with someone who's in a monogamous relationship. Yes, they will ask you.
• A celestial succubus will never admit to being an angel. You cannot give consent to someone you know is an angel, due to the power imbalance.
• A celestial succubus will never lie. Lying is a sin. Lies of omission sometimes count, but in most cases responding to a question with a flirtatious smile and a playful shrug is acceptable.
• A celestial succubus will always make clear exactly how long-term the relationship is going to be. Most of the time, this will be a one-night stand, because succubi are very busy. Very rarely, it might be a matter of months. Succubi never enter serious, long-term relationships, because that very easily crosses into the territory of a lie of omission. 

One thing the pornographers don't always feel the need to mention is that succubi rarely have sex with attractive humans. It does happen, of course, but less often. Succubi spend most of their time bolstering the self-esteem of ugly losers and making sure the old and dying have one last thrill before they go.

Succubus Tactics

Succubi are very good at what they do. They have had a long time (Millenia? Eternity?) to practice, and are extremely hot. They are masterful flirts. They know exactly how to make a person feel good about themself. They can happily pander to any kink or worldview, and are very good at figuring out what a person wants of them. Like all angels, they love everyone, and they aren't afraid to confess their love if they think it's the right thing to do.

Succubus Psychology

Succubi have no sexual preferences. They love everyone equally. They think everyone is beautiful. They know that some people are worse sinners than others, but that's not really that big a deal. Even the holiest paladins and clerics are fundamentally wicked and sinful in the eyes of the angels. 

Succubi do experience sexual pleasure, because otherwise they'd have to lie about it. They don't particularly enjoy the feeling of pleasure, but they don't mind it. Like all angels, they feel incredibly happy and satisfied all the time, because they are always living perfectly in line with their morality.

Aasimar

Whenever possible, succubi avoid the use of contraceptives. This is because, as the one kind of angel that has sex, they are the only way for aasimar, or half-angels, to be created. Aasimar often never discover the truth of their parentage. They are always raised by the human parent (This is always made clear by the succubus beforehand). Aasimar are powerfully inclined towards good, and are often stronger, smarter, and tougher than their peers, and sometimes manifest additional celestial powers. A single succubus can sire countless aasimar, and these heroes likely do more good than the succubus themself.

(Alternatively, maybe the offspring of a succubus and a human is an angel. Maybe this is the only way for new angels to be made).

Succubus Bodies

Presumably, succubi vary as wildly as human sexual preferences do. Male succubi can sire significantly more aasimar than female succubi, but then again there are probably more straight men who need to be tempted away from evil then any other group, so I guess it all balances out. Maybe succubi can shape-shift, but I think that gets closer to lie of omission territory.