Neither are the fifty billion early access games on Steam that people buy, fully aware that they are getting a game that is still in development. What do you imagine the reaction would be if every single Steam early access game's "Buy" option was removed and replaced with a "Pay a montly fee to access this game" option?
I absolutely agree, but porn games are such a small niche, that most real devs will never touch it with a ten foot pole.
Which leaves you with mainly first time single devs + maybe an artist or two, who often would rather do more respectable stuff, too.
And as they lack funds from previous projects and lack experience and tools, they can't get money upfront from a publisher/bank. And self publishing on steam is too much work/risk early into development (and since Steam doesn't want to put in the extra work to comply with non-US law, you usually can't even sell your game in countries like Germany).
That said, some of the more successful matured projects will occasionally break into the steam game at some point - but most projects will either never be finished or never get released on steam.
So if you're a perv and someone is catering to your needs - and you want them to continue, there sadly aren't many options. I support some projects from time to time, but I'm not spending ~$100 per year on a single game in very early access.
But I'm happy some people are.