Why do you think sex is needed in relationships?(besides marriage)

I've never been in a relationship or anything, so excuse my ignorance if you will. Does it hurt you or bother you that after you gave that person something that might or might not be special when you guys broke up? Is it hard pill to swallow? After all your experiences, can you guys come to a conclusion whether it's worth it or not? Thanks. 
Oh and don't be rude. 😒