Question:

“Fornication in the Bible seems to refer to adultery, incest, homosexuality, and bestiality. I don’t see anyone being called a sinner for engaging in premarital sex.”

Answer:

In Bible translations and Bible commentaries and so on, when they talk about premarital sex they often refer to it as “fornication.” Here’s what the word “fornication” means:

• “Sexual intercourse between a man and a woman who are not married to each other.” (West’s Encyclopedia of American Law )
• “1. Noun – Unlawful sexual intercourse on the part of an unmarried person; the act of such illicit sexual intercourse between a man and a woman as does not by law amount to adultery.” (The People’s Dictionary )
• “1. voluntary sexual intercourse between two unmarried persons or two persons not married to each other.” (Dictionary.com Unabridged )
• “Fornication: Sexual intercourse that is “illicit”, outside of marriage.” (Medical Dictionary )
• “fornication n. sexual intercourse between a man and woman who are not married to each other.” (Law Dictionary )
• ” VIRGINIA BEACH — A lawsuit that accuses a Virginia Beach man of intentionally passing herpes to his lover may have implications for state law on fornication between unmarried adults.
  Attorneys for a woman identified only as Jane Doe filed suit this month in Circuit Court, claiming that a Virginia Beach man gave the woman genital herpes after the two began having sexual relations in April .” (SUIT SAYING MAN SPREAD HERPES COULD AFFECT FORNICATION LAW )
• “Main Entry: for·ni·ca·tion
  Pronunciation: \for-ne-’ka-shen\
  Function: noun
  Date: 14th century
  : consensual sexual intercourse between two persons not married to each other” (Merriam-Webster Online Dictionary , emphasis added)
• “NOUN: Sexual intercourse between partners who are not married to each other.
  WORD HISTORY: The word fornication had a lowly beginning suitable to what has long been the low moral status of the act to which it refers. The Latin word fornix, from which fornicti, the ancestor of fornication, is derived, meant “a vault, an arch.” The term also referred to a vaulted cellar or similar place where prostitutes plied their trade. This sense of fornix in Late Latin yielded the verb fornicr, “to commit fornication,” from which is derived fornicti, “whoredom, fornication.” Our word is first recorded in Middle English about 1303.” (The American Heritage Dictionary of the English Language: Fourth Edition. 2000. , emphasis added)
• “c.1300, from O.Fr. fornication, from L.L. fornicationem (nom. fornicatio), from fornicari “fornicate,” from L. fornix (gen. fornicis) “brothel,” originally “arch, vaulted chamber” (Roman prostitutes commonly solicited from under the arches of certain buildings), from fornus “oven of arched or domed shape.” Strictly, “voluntary sex between an unmarried man and an unmarried woman;” extended in the Bible to adultery.” (Online Etymology Dictionary , emphasis added)
• “The Latin verb fornicare, which is the source of English fornicate and fornication, is derived from the noun fornix, ‘arch, vault, arched basement’. Because brothels were sometimes established in the Roman vaults, fornix itself took on the sense ‘brothel’ and the derived verb fornicare was used with much the same meaning as modern English fornicate. The noun fornication appears in English at the beginning of the fourteenth century, some two hundred and fifty years before the verb fornicate. In 1303 Robert Mannyng of Brunne in his penitential manual Handlyng Synne did his best to define the noun with the utmost discretion, and though his fastidiousness resulted in some vagueness it is dispelled in part by the context: “‘Fornycacyoun’ [ys], whan two vnweddyde haue mysdoun.” (“Fornication” is when two unmarried people have done wrong.)” (The Merriam-Webster new book of word histories, p.182-183 , emphasis added)

So “fornication” refers to voluntary sex outside of marriage, and the earliest recorded use of this word in English (with the meaning of premarital sex) was in the early 14th century, around 1303.