Either your save is corrupted or you made a bad edit in it yourself. sex_times is the variable in your save telling the game how many times you've completed the Bedroom Endurance Upgrade. It's stored in a nested table at: game_res > upgrades > sex_times. It should be set to an integer value between 0 and 3 I believe.
You can use Notepad++ with JSON viewer to quickly navigate and edit the game's save files (which are stored in ::User/Appdata/Roaming/Strive 2).