Sorry, I somehow missed some replies.
Maybe the dragon butter is hallucinogenic
@TheChip IDK if this is what you meant, but I've decided that they are persuaded into a ritual that uses drugs. It's only once they're off their tits that the person leading the ritual tells them to do the mum-necrophila.
I told my friend they could maybe be persuaded into it while they were on drugs. He said 'I suppose they would have to be.' 
Can't you write about kittens instead?
@PerpendicularVincent There is a person with Body Integrity Disorder (they think they should've been an amputee.) They have a plushie kitten which is their imaginary friend.
How can a man shag a pelvis? It's much, much bigger than a cock. You're basically just thrusting into a large gap which has a pelvis around it. The issue is going to be getting an erection more than anything else. I think this needs rethinking.
@foxhat I suppose they could angle/go in at a height where they could rub against one of the pelvic bones. It's pretty gross writing about the stuff, but it seems like part of my life's work. I guess it's partly driven by reducing some of my bipolar meds (with doctor's agreement.)
After they're persuade into the mum-sex, I left it all 'behind the bedroom door,' saying just 'you try not to think about it afterwards.' So I don't have to go into the details of what went on.
Skeletor is hot! Way hotter than he-man!
@CandyLeBonBon Whatever floats your boat I guess. She-Ra looked ok. Maybe I should write someone into the book who fancies Snarf.
Why are you choosing this as the thing the group have to do? Like you're the writer it doesn't have to be this. So why this?
@Okokokbear The plot is randomly generated. I researched a list of rare genetic symptoms, neurological/mental ones, kinks and bizarre spiritual beliefs. Then I put it through a random list generator, so my job is to string together the randomly-generated elements.
I'm not in control. It's postmodern. The title of the book is 'Sick Kink of God,' because they are sick, kinky, and spiritual, and their sickness (and arguably the kinks) are randomly assigned to them at birth. It will also have a twist at the end 
Wouldn't limbs and bits just break off of the skeleton if the people actually agreed to the crazy plan and did unspeakable things?
Maybe they would have to be gentle IDK. Bits detatching needn't stop them. I think the pelvis would be intact, though the femurs would detatch.
A PP mentioned a film where someone mentioned skullf*cking, so I suppose that's a possibility.