if sex is not love, why do we complain of cheating?

We always tell girls, sex is not love. We always tell them if he loves u he will wait for u until he marries u.
This friend of mine now asked, if having sex with someone does not mean the person loves u or u love the person: why do people cry, feel bad, heart broken when their boyfreind or girl freind goes into intimate adventure with another person? Since then I have been thinking.