Over the course of a month, the reality cascade has spread to every person on Earth. Mankind is now and always has been Dorsekind.
First Big Change: No More Children
Dorses can reproduce, but the offspring will stay in a small, premature form for a maximum of seven days before growing into a young-adult dorse. It has grey, slimy skin with no hair, no reproductive or even digestive system and stinks to adult dorses. They feed off what's left of their placenta to develop their adult bodies. After that, they can get in on the fucking.
Second Big Change: Anatomy
Aside from the obvious, strong changes happened to dorse immune and digestive systems. First off, they have tough bodies, but are not invulnerable to damage. Dorses don't feel pain as intensely as they did as humans, often finding pain arousing. Their immune systems are nearly perfect, blocking most toxins and diseases. They will eventually die as they get older and weaker-though their average lifespan is 350 human years.
Second, dorses have two-entrance-no-exit digestive system, which is where the anus acts as a second mouth with a straight path to a giant gut in the center of their body. The stomach holds a hydrochloric acid much stronger than a humans, able to dissolve almost anything. The inner walls of the stomach is lined with villi that absorb nutrients and minerals, meaning a dorse can live anywhere on the planet. The closest thing to waste a dorse produces are gasses that are produced during digestion, which come out as burps and farts. Luckily, the gas carries no harmful microbes. At worst, it's a little smelly.
Biggest Change: Societal Structure
Dorsity (Dorse's Humanity) is a largely free-use-wanderer society where most property and space is shared. There are towns with buildings to stay/work in and tools/equipment to use for work. As long as the place/thing you find is empty/isn't being used, or whoever's already there is willing to share, you can use it for whatever-just keep any damages to a minimum or you will be fined something. The economy has largely based on trading goods and services (with most of the services being sexual).When borrowing something from someone, most dorses make an effort to return it.
It's a generally peaceful world where most people are kind to everyone. It's the end result of a species dumb, horny, free-use horse/human chimeras being to only sapient life on Earth.