Short story read in an anthology in UK maybe ten years ago, I can’t think of other stories in the book, or recall any details of the book.
Protagonist and his future people live in massive space habitats with artificial suns, they get nourishment by lying out in the solar light for a couple hours daily. Big black photosynthesis cells bloom open from their pores. Mankind has been gene modded like this for thousands of years.
One day he finds an ancient book about Christmas and gets intrigued, his girlfriend and another couple also enthuse over the “ancients celebration rituals”
They decide to recreate a Christmas feast but first they have to learn about kitchens and cooking etc. The fabrication machines do them an oven and a hotplate range and tableware and they spend a while familiarising themselves with it all.
They have ordered loads of foodstuffs from old patterns still stored in the main computers and the big day arrives. The previous evening they had the autodoc turn their metabolism back to our normal.
Christmas morning protagonist and girlfriend are busy cooking and the other couple arrive to help, as the morning progresses they find themselves drooling at delicious aroma and they keep snacking with appreciative moans.
They keep saying “why would the ancients give up this pleasure?” Finally they sit and have a massive feast of traditional Christmas stuff and again they’re giving it “why would the ancients give up this pleasure?” They finish with pies and they’re like moaning with fullness when the other man looks around puzzled at his bottom. He breaks wind and the smell gags them all out.
Thirty minutes later they’re all screaming in total horror as the first poo for millennia happens to them all, in a culture with no toilets or toilet roll