Can of worms and then some.
Firstly Steam does not sell pornographic games. Dating sims yes, outright porno like TiTS no. I believe this is a situation that will change in the next four or five years, and given their penchant for expansion and the unrealised potential for profit I'd bet anything Valve have already tentatively looked into it. But it's laden with a lot of issues on the distributor's side that aren't easy to resolve.
Secondly - a different transaction model would have to be designed, and really Fen would have to come at TiTS in an entirely different manner than he does now. Oh god, I'm just imagining the NT thing and him rambling off to write another furry futa waifu instead of space mechanics if he was an "official" game dev with TiTS in Greenlight/Early Access, it would be an absolute bloodbath. With an official platform, a bigger audience and greater potential for profit comes serious responsibility and much closer scrutiny, and the chance that even if you do diligently keep to a timetable and do all the things you said you would, people will still review bomb you. I'm not saying Fen doesn't work hard, but the leeway he has now suits how he goes about things much better than in that scenario.