Under the functional perspective of sociology, you could say that marriage or a relationship provides safety and continuity. You don't have to go out looking for sex, and the diseases that multiple sex partners could bring. Sex is a basic need, as noted in Maslow's Hierarchy of needs right after air, food, and water. The next need is sex. Having a relationship "guarantees" that this basic need is being met, without stress and having to hunt. At least you'd hope that it guarantees that. I feel very sorry for sexless marriages.