Hello, this button is for connecting external fleshlight device to the game, specifically "Lovense Max 2".
Recommended: Tap this button > (a popup appears) > tap ? at the top left of the popup for instructions > now you can just tap in this popup to scroll instruction pages (contains text description and walkthru screenshots).
In case you assumed this is Futa mode : futa can be turned on by opening shop menu (The green Dolar sign button on the right), and tapping on the futa.
Cheers,
~NK