Many people have learned that sex outside of Gods arrangement (marriage) does not lead to happiness...it often makes people unhappy and brings trouble to their life, ie unwanted pregnancy, sexually transmitted disease, lack of self respect
and it can often cause relationships to sour. You should know that most woman seek a partner who wants to stay with her...if your idea is only casual sex, then she may feel betrayed and used by you at a later time... and thats even if you've told her you only want casual sex because a womans emotional makeup makes us prone to attachment.
Casual sex and premarital sex aren't necessarily the same thing.
Unwanted pregnancy, stds, etc. only happen with irresponsible sex, and I'm not sure what the lack of self respect comes from.