SexSide

From the letters page: Fantasy Games's SexSide is a visual novel made with Unity, currently finished. Themes: horror, romance and harem. Presentation uses 2D art and animated scenes. Available for Windows, roughly 657 MB.

From the box: “You finally have a girlfriend… inside a video game! But she won’t let you leave. You’re trapped in SexSide, an adult game of alternate versions of your POSSESSIVE virtual girlfriend.”