While I can see it with demonic scions I can't with Changelings, at least not every time. Sure, trolls and ogres, but people could be easily swayed by a particularly lovely sidhe into a night of passion, especially if they didn't know the true nature of their lover. With the right glamour it would work for uglier Fae as well. Even the ugliest troll might be looking for a little love in the mortal realm!
Remember that the Immortal Beings (Angels, Fallen, Fey, and Demons) are all 'locked in' their ways
- in other words, they can't do anything outside their Purpose, or against their Nature.
A demon that does nothing but eat people is not inclined to mate with a Human, unless
forced to do so by another Human.
And it's going to hate all of them: Master, Mate, and Offspring - and want to eat them all, and at the first possible moment.
The problem with most of the 'Immortal Beings' (Fey, Demons, etc) is that they don't really understand Love.
Lust, yes. Power (as in 'a controllable agent in the mortal Realm'), yes. Legacy (powerful descendants), yes.
Running a Deception upon a mortal, for the mere fun of it? Yes, especially the Sidhe: The longer lasting the Joke, the better.
Scions of Fey and Demons are scary to both sides of their Heritage, simply because they have great power and are unpredictable due to their Human Free Will.
Angels are pure Love, just as they are pure Spirit/Soul.
Scions of Angels were created by and for Love, but caused the Angel to Fall.
Fallen are exiled Angels, but still remember pure Love, but may no longer seek to help it flourish.
Remember that only the Denarians (30 silver Denari coins) are able to directly influence Humans, by tempting those that take up their coins - if the Human gives too much Will/Choice away, and they can dominate you. I'd think that even these trying to mate with Humans would still rarely produce Scions. Mostly because they have to figure out how to bypass their Mortal Host's 'limits'.
Scions of Fallen are every bit as scary as Fey/Demon Scions, and for the same reasons.
All the Nephilim are akin to hated half-breeds: they get flack from
all sides, simply because they were
born.
And the Heavenly Hosts don't respond to Nephilim any more they they do with any other Mortal Human, which means that you end up with a lot of them with the "
Angstified Teenager" syndrome.