God, Who determined the punishment for sexual immorality, has given reasons for making the punishment so severe in the Bible, including this description of our relationship with Him: In the OT, God described Himself as the husband of Israel, and in the NT, Jesus is the bridegroom of the church; so, when the Israelites, or the Christians, practice sexual immorality, they commit spiritual adultery, turning their backs on God and choosing someone else to "love."
People have judged some sins as worse than others, but I'm not sure God does. Is homosexuality more sinful than adultery or incest or forced intercourse with adults and children (female and male!) enslaved in brothels? Is abortion more sinful than murdering a 7-year-old or a teenager or an adult? Is raping a woman more sinful than torturing a man? These are all horrid evils, but I cannot say that one is more sinful than the others. And the Bible does not tell us for sure that God thinks so either.
As we read Paul's warnings about sexual immorality, we should look to ourselves and examine ourselves and our actions to see if we commit any form of sexual immorality, and if we do, we must repent and cease that behavior immediately. Only then may we exhort others who are committing sexual immorality to repent and stop that behavior, and we must do so with agape-love as our motive.