Just gonna throw this in here:
Lostlegends The post
alex2011 probably talks about is
this one.
Yes I understand the issue regarding Alignment/size and animation, and I think the dev had the right attitude. Lifeplay for instance went the other way and made single size models, which in my view is boring, and limits the content you can do.
However there are other choices, IK which the dev chose in this case is a pain in the arse to work with.
Thats why the male looks like he's having a fit during sex, and still nothing lines up properly.
A simpler way(That still has a few issues) is to use standard animations on both players.
You then rig the models within objects, centered around there genitals!
So your npc model for example, has 3 empty GameObjects's on its body, Mouth/Vag/Ass these spawn with the character.
Your player also has these objects.
Oral sex, Set position of player by the dick 'GO' to the npc mouth 'GO'. This then has perfect alignment requires No 'IK' and has no male having a spasm.
This is the way most RPG's equip items when made in Unity, there are tones of vids etc explaining the basic principles.
The only slight drawback is both characters wont align perfectly with the floor, but they dont now anyway.
But this is just the way I would do it I suppose, so feel free to carry on with with sex spasms lol.