Jon, I agree with the others re. it being slightly overwritten, needing paring down to sharpen the focus, getting more in the protag's head, adverbs, clichéd phrasing.
Apart from that there are a few bits which confuse me or jar slightly. Just taking the opening section:
"Does it bother you?"
Membra rolled herself onto her side and looked at Custos as he glanced up from the book he was reading. He looked back at her quizzically.
"What?" he asked.
Too many looks. I would trim to something like:
Membra rolled herself onto her side and looked [how does she look? - I'd change this for a stronger verb which helps convey how she's feeling] at Custos.
"What?" He glanced up from his book.
Membra did not answer. Instead, she manoeuvred herself onto her back again and stared down at herself. Every sweat-dampened crease in the coarse silk beneath her was a maddening niggle of discomfort. The night was sweltering and both she and Custos were lying on top of the bedclothes to make the most of the breeze from the open window.
Cut first sentence – we know this from the lack of her dialogue – or if you want to highlight it I’d just go for ‘Silence’. ‘Manoeuvred’ seems wrong here. I don’t know how her injuries have left her body exactly, but I’m imagining the getting onto her side from her back is harder than the reverse movement so I’d imagine the two verbs reversed too? ‘Coarse silk’ seems to be a contradiction to me – isn’t silk soft? It makes me pause and distracts instead of drawing me in to the scene. I might trim the last sentence. We know it’s hot because of the previous ‘sweat-dampened’ reference, perhaps just reference the breeze from the open window?
This is very close editing, I know, but I think it would improve things massively if you went through and checked what is actually needed/repeated/already inferred etc.
I won’t carry on with the rest in such detail, but a few things which jumped out as confusing/jarring are:
subtle whorls of fine down > women's hair doesn't usualy 'whorl', it's not long enough
planes and angles of her body > seems too blunt for a body, a body is usually made of curves
overuse of 'would' in the seers section
the descriptions of the other lovers are a bit too clichéd - skinny thief, burly smith, mouse-like scrivener, sea-captain with a husky laugh... Make it a bit more surprising, shake it up a bit so they're not stereotypes
Custos rolled himself to face her. > I had pictured them with him on his back and her curled into his side, on her side. So the movement of him coming to face her, especially taking her disabilities into account, would be more complicated than a simple roll, wouldn't it? Now I'm taken out of the scene because I'm confused as to what I'm seeing
fingertips carefully tracing the contours and creases of each and every scar > I can't picture the scars - can you draw a picture for me? what shapes do they paint? they seem to be important
She arched her back...in anticipation. > cliché, I would rethink the whole sentence trying to rid myself of the standard ways of describing this
His cheeks were damp with sweat, or tears, she could not tell. > this surprised me, is he crying too? I thought only she was. Would he be crying? He sounds very solid to me
and their tongues gently entwined > really? sounds like they have really long tongues to be able to intertwine them!
quiver of delight / wild little cry almost like a sob / crescendoing rhythm and the ecstasy / coarse tangle of his chest hair / she shivered with pleasure> clichéd phrases, I’d rethink and rewrite with freshness
planes of her hips and buttocks > do you mean 'plains'? Planes seems too flat again for a body description
growled / pleasure point > ditto what L said
Felt him slip inside her. > this doesn't ring true for me, without going into too much detail (ha, ha) I think the verb makes it too easy
He was as shaggy as a bear and smelled of soap and recent sweat, musky, male. > apart from being very clichéd, I can't believe the dual soap/sweat smell - it's either one or the other, isn't it? clean sweat doesn't really smell much
I hope some of this helps. I’m aware of sounding overly negative – there are phrases and turns I like too but I figured it’s the jarring bits that are more helpful. And I should say that I’ve never tackled a sex scene (writing MG/YA) and imagine it to be quite complex, especially from the opposite sex’s POV.
Good luck with revising ;-)