Yes, it is a Sin.
1Corinthians6:9-10
Do you not know that the unrighteous will not inherit the kingdom of God? Do
not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor
homosexuals, nor sodomites,
nor thieves, nor covetous, nor drunkards,
nor revilers, nor extortioners will inherit the kingdom of God.

Aside from sinning, you'll be prone to sexual diseases like STD, Gonorrhea, HIV and AIDS. Prevention is better than cure so be self controlled. Just don't think of lustful things and shift your mindset to what is good, noble and righteous..

Some people do not believe until they the consequences of their sinful actions. And many pretend not to believe because they are still into it, trying to justify their actions. Even morality and values education indicates that fornication and also adultery are forms of immorality. Thus, no matter where you look at it, it is still a sin in the eyes of God and in the eyes of man.