Yes, it will be distributed by third party means (no idea how atm) and will not be directly advertised, to avoid having the "explicit sexual content" (or whatever it's called) tag on Steam, but the game will still have "sexual themes" due to lore entries and some background action...the adult addon will add back in the missing pieces and filling these gaps (this has been the plan since the start and one of the reasons as why the H-scenes are "delayed" for later in development)...I'd wondered about that, since last I heard, Steam isn't fond of "non-con" content in the games it hosts. So, that content will be an external site download, akin to downloading lewd mods for games like "The Elder Scrolls" and "Fallout" series?
It might be posted on nexus mod, since the way the game is being developed will allow for extra packages to be deployed and the dev has been thinking about mod support for more than a while now...