1Co 7:2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
Gal 5:19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, Gal 5:20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Gal 5:21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.
Strong's G4202
πορνεία
porneia
por-ni'-ah
From G4203; harlotry (including adultery and incest); figuratively idolatry: - fornication.
Outline of Biblical Usage from Blue Letter Bible
- illicit sexual intercourse
a) adultery, fornication, homosexuality, lesbianism, intercourse with animals etc.
b) sexual intercourse with close relatives; Lev. 18
c) sexual intercourse with a divorced man or woman; Mk. 10:11,12
- metaph. the worship of idols
a) of the defilement of idolatry, as incurred by eating the sacrifices offered to idols
Fornication is the word you are looking for, sex outside of marriage between two unmarried couples is fornication the greek word is porneia. Yes fornication will keep you from inheriting the kingdom of God according to Gal 5:19 and 21. It is very serious.