Biblical Doctrine forbidding sex outside marriage

How do we get that “Sexual Immorality” equals “Sex outside of marriage?” I cannot find Old Testament law stating it is forbidden. I can only find where sex outside of marriage, along with sexual immorality,gets termed as fornication. I also hear the word Porneia translates as sex outside of marriage. My Strongs defines it as Deriving from harlotry (includ. Adultary and incest); fig. Idolatry