Is fantasizing sin?

If you mean fantasizing (dreaming) about engaging in sex with someone you’re not married to, then those thoughts would dishonor marriage and God (Hebrews 13:4). This is not who God made you to be. God calls all people to believe in his Son as their Savior. When the Holy Spirit creates that saving faith in your heart, you become a temple of the Holy Spirit, which will compel you to honor God with their whole being—mind, soul and body. So yes, it is sin – but it is also a sin Jesus paid the price to forgive.