Lack of chemistry
After sex, oxytocin in women increases the need for closeness and cuddling, and in men increased testosterone levels. He unconsciously wanted to do something productive for him to raise his level of testosterone, which is lying around.
When a man has sex with you too soon, he feels guilty and wants to escape from the crime scene. He knows that you used and just wants to get away. Cruel but true.
You give too much
Men do not fall in love when they have, they fall in love when giving. He falls before sex, not after it. If he thinks he deserves to be with you through the effort to put a courtship, it will remain with you.