In order to get this achievement it can be quite tedious, the only way to getting it is restarting the game over and over till the first time you can use the Girl Finder, When you can you want to search for the pair of Ashley and Lillian. Once you get them you want to Date them first in the afternoon then date someone else in the evening or alternatively you can go to the Hotel Room and sleep having Kyu wake you up. then once your at Night you want to date Ashley and Lillian again. If you complete this date without failure you'll unlock the achievement "Unchaste With Haste"