The basic principle of material attraction is sex. We have organized sex life in many social conventions. Marriage is a social convention that gives sex a nice finishing touch, that's all. Sometimes it is said that marriage is legalized prostitution. But for keeping up social relations one has to accept some regulative principles, some restrictions on sense gratification. Therefore civilized human beings recognize that there is a difference between sex in marriage and sex outside of marriage, which is just like sex between animals.
Srila Prabhupada, quoted in Journey of Self Discovery 3.2: Liberation to a Higher Pleasure