You're right, the game is very easy at the moment. I've been thinking about this and what I could do to add some challenge for players who'd like that & also make it feel a bit more realistic.
I plan to add difficulty options, set when starting a new game & changeable at any time in settings. The current version would become the new easy mode. Medium and hard would add more pushback from girls, a new dieting system and the possibility that they get upset with you if you go too hard with the feeding.