It depends on the tpe of adult game, actually.
Any VN piece HAS to rely completely on the storytelling aspect, and choice freedom is limited (if it exists at all), on the choice of who you want to screw first, literally, figuratively, or both.
And, that's why around 50% of the adult games are, in fact, VN, or "almost VN", based on linear, or almost linear progression.
BUT!
We have actual, very successful examples where freedom of choice, or, as it has been mentioned already, environmental storytelling, worked wonders, even on an adult, porn game, that you're expecting to play one-handed.
Games like bonetown, for example, made exploring as much fun and enjoyable as screwing the next hot chick, if not even more fun.
And, here lies the real issue with that, just like desmosome said above. Development resources.
It's much easier to create a storytelling experience, than it is to create a sandbox environment, even if both are fully animated at the player's irect input (i.e. picking the poses/animations, and scenery, speed, etcetera).
The storytelling option has fewer characters, a theme that may limit the available pose selections, and very finite choice directory trees. For example, in a femdom themed title, anything maledom is out of the question, as well as most "rough fuck" variants, and so on.
But, on a free choice scenario, everything goes with everyone, and trying to add limitations per random NPC will make the overall game even more complicated, and resource hungry than it would already be, unless a team of developers work hard to optimize everything to make it playable