Because I hope not.

Not that I'm calling for the game to be removed or anything, I just hope it's not a sign of more to come in the category of Henti games and stuff. This appears to be GOG's first AO game.

Succubus is not what I would consider an "adult game" though - it's a FPS with some adult content.

There are quite a few (censored) games with adult content on GOG already, btw. The floodgates have opened a long time ago and we've yet to have a "furry invasion".

And even if we did, nobody's forcing you to buy anything - much like any other game type/genre that you're not interested in, you can simply ignore it.

Well, tough luck.
For example, Subverse is officially confirmed to arrive later this month.

Why? There are people here who want to buy more AO games, and, unlike Hitman Lame of the Year Online Edition, AO games are not incompatible with GOG's purpose as a DRM-free store. GOG was not forced to accept that Hitman game, and you are not forced to buy any AO game.

Also, we had adult-oriented (not as explicit) games with "mature content" on here for quite a while already:

https://www.gog.com/game/lula_the_sexy_empire
https://www.gog.com/game/biing_sex_intrigue_and_scalpels

Is this Succubus really more "adult" then say Witcher series?

It's certainly more "adult" in the explicitness of its depictions of adult-oriented content.
Where CD Projekt RED usually chickens out and uses elaborate camera angles to not go into too much detail of what's going on below the waist (except for butt shots) Madmind Studios obviously goes all out (and in) instead.

Succubus is about as "adult" as a 13 year old wearing a black leather trenchcoat and listening to Limp Bizkit.

That, and your hopes have already been dashed, given we already have LSL, the aformentioned Lula, and Huniepop & Huniepop II: Stereotype World Tour.

While, I do hope that GOG will make choices which reflect actual maturity, I have sadly resigned myself to thinking of GOG as bizarre starfish aliens who only communicate using deflected quark rotation.

If GOG adds an option to show/hide adult games (hidden by default), I see no reasons not to include such games in their catalog, completely uncensored.

As for Succubus, from the game footage I've seen I don't see what's so adult about it. Can somebody explain what exactly in this game makes it an adult game?

The wanton edge and nudity. Tyrannically puritan societies see an exposed set of breasts as a big no-no. But as I described, this is a teenager's (who needs to visit their therapist) idea of mature.

Succubus is from the same team that brought us the agonizingly bad Agony, another straight faced and completely unaware "mature" game.

You know, the sort of thing akin to Shadow the Hedgehog.

I see, it's doesn't deserve more than a 16+ rating then, there's nothing adult about it.

Yeah, but that's rating systems for you.

Yes, there's a difference between games with adult content and games that are adult content Well it's the first game on GOG to have the "are you mature" question pop up. There's also this video here. https://www.youtube.com/watch?v=kg5AHeU9q40

Despite the description, it is still censored for YT. But yeah this is pretty adult I'd say.

Main issue here is that different regions have different views of what "Mature Content" is, some places consider nudity and sexual themes mature but are more laxed on violence and swearing, while others may have issues with the same violence (blood/guts etc) but are less inclined to worry about depictions of nudity, while other places have issues with both or even consider sexual orientation enough to land "Adults Only".

I'm going to stop now as I'm sure this is leaning political.

It was not the first game to have the "are you mature" question, I first seen this weeks (months?) ago. And, I could be wrong but I'm pretty certain most people in Europe would not consider this to be adult content, I certainly wouldn't.