I'll be curious to Phantom's answer here, but to my understanding, all the mantic stuff is optional. The only thing that I'd personally love some clarity on is with regard to Lily. It's been discussed on the Steam messageboards as well repeatedly that people want to know if any of the mantic stuff with Lily is made to be in-universe-canon with regard to her fucking mantics to any degree as that would ruin her as a waifu for a measurable chunk of the (to borrow your terminology) "mainstream" customers, myself included. So far we haven't been able to get Studio FOW to directly answer that question.
Yup. A whole lot of the people on F95Zone seem to believe that everybody just wants tentacle rape, but F95Zone is a hardcore porn game forum for people who want that kind of content, so obviously everyone here is biased towards more niche fetishes.
With how incredibly big the game has gotten, I doubt Studiofow knows that most of the people on those steam forums are mostly vanilla dudes who wanna bang their "waifus". But all the kickstarter mangas and such are all, well, typical studiofow alien/monster rape stuff. Feels like there is a disconnect there, especially with how little marketing the would-be protagonist is getting.
I am pretty sure Lily's stuff is in-universe canon though (I don't mean the doujinshi, but her fucking aliens in general), as her entire personality seems to be "I make aliens to fuck them.". No doubt Steam is gonna get pissed about that. And I'll be disappointed, but I wasn't really keeping different expectations.
I believe you can choose not to see her fucking the aliens, but she does fuck the aliens. You can hear moans from her door or something. Idk when or where I read that though so this is pretty much just guesswork. I do hope we get some sort of option that goes "Remove all alien content" or something. Like a tick in the menu.
I'm just wondering about the other waifus and all. A l l of the kickstarter updates are about the mantics (or the combat). The fact that we still have nothing on the protagonist is worrying.