I have a feeling that the 19 year old girl dissappearing may have more to do with her age, and yours, than you giving her amazing sex. In my old age I've found that keeping women from that age is difficult. Their not only in their physical prime, but if they're social then they're partying a lot with friends, busy with school, etc. The sex may be great, but she can get good sex from many men. Without more info that's all I can guess, and am probably miles off the mark.