Both of you were into the relationship for fun, then you need to understand that it cannot go long way. Particularly this is more so when you are aware about her promiscuity. You have to manage your parents, respectively; by whatever way you think the best e.g. faking break up etc. But clear opinion is not to marry with her, husband can never live comfortably with promiscuous wife even if it is her past. You and other readers may find it bitter but this is the fact of life.