Like this

Like the first but the world they live in would offer wide beds to accommodate for their elongated needs.

I would assume the human part would be able to do human things like bending over to pick up things from the ground with their human hands

Basic centaur knowledge.

Horses don't sleep on their side, except for brief periods. They lay down and curl up like that cow illustration another poster commented and doze away. They sleep standing up, except for brief periods of time when they do lay down for heavy REM sleep

Basic horse knowledge

So my take is neither A nor B but an accommodating solution where my centaur partner has a bed big enough for them. The depiction has the centaur as having a very stiff spine and we can assume it needs more sleep than a horse. The human part having a human brain and all (basic centaur knowledge)

So a big comfy bed we can both fit on with a million pillows (you can never have enough) and spoon inside my partner's upper legs like A but their lower half would be nestled upon the bed like B, with some pillows to help cushion the hooves. They're warm but a bit rough (basic horse knowledge). Also horses have a body temperature about higher than ours so they feel like a furnace (also basic horse knowledge) so not many blankets would be needed

If it's a fling I'd just go back to theirs

Thank you for attending my Ted talk. I love horses and centaurs and have thought way too much about them both. So far my breakfast has been booze and weed