physex won't be on steam in its current form, because it is an ever-growing engine/project rather than a traditional game, and it needs ongoing funding to keep expanding.
that said, I do plan to make an actual game built around it, and that one will be released on steam. eventually, I’d also love to make the engine available to other developers.
for now, the goal is to keep adding the features, ideas, and requests supporters come up with or vote on, and keep pushing it towards becoming the ultimate sex sim