Of course it is.
Matthew 19:4-6, 8-9 NIV
Havent you read, he replied, that at the beginning the Creator made them male and female, [5] and said, For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh ? [6] So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate. [8] Jesus replied, Moses permitted you to divorce your wives because your hearts were hard. But it was not this way from the beginning. [9] I tell you that anyone who divorces his wife, except for sexual immorality, and marries another woman commits adultery.
Sex before marriage is a gross deviation from God's plan, therefore immoral, and a sin.
1 Timothy 5:1-2 Do not rebuke an older man harshly, but exhort him as if he were your father. Treat younger men as brothers, [2] older women as mothers, and younger women as sisters, with absolute purity.