Erhh I understand why we can't masturbate in the game since it would be weird then for why you can't do so without any sperma and would either have to be an specific spot to interact with or another keybind that really isn't needed. (It's also really easy to gain sperma, though impossible to gain it without switching to a new screen since you need enemies)
Plus it may have been planned but was cut out of the last and final update due to the Dev being forced to do so before moving to making their next game due to RL stuff.
But that's why I modded the game to make it so the struggling is a bit harder for those seeking it along with making journey to the core section include EVERYTHING from prior to it, also made it so money was spent for ammo, armor, hp and key max buffed. Oh and some other minor changes to enemies stats like range and speed.
Doing that again though in Unity (the engine they've switched to) will be harder however due to it being less modifiable if you only have some slight code knowledge since the engine is a bit of a mess when I've looked at a few other games. (which is in part due to the Devs themselves also not notation'ing everything correctly or at all) Also it's a bit harder to bug test I believe.