It´s about how the human mind works. If you have sex with somebody who is relationship material on the first date, you are sending him/her the opposite message. If you want to have something serious, leave sex for (about) the third week of dating. Just because, when somebody puts the "fling" label on a date (or, on yourself), it will be VERY hard to make him/her change its mind. If you want your date to consider you as relationship material, be patient.