Why my game saved without my permission?!

^ Yeah, like what the others said there's the autosave feature. Technically, I think they have changed how the autosave file opens. It should not load up by default when you click on that big play button. But just in case, next time when you don't want to save your game, go back to the main menu and hit the floppy disk button at he top right to open your manual save.

If you have forgotten to that again and for some reason it opens the autosave, just exit again without saving and open the manual one. The game will never overwrite the autosave file over your manual save, so you should be okay.