When Paul was giving his opinion he prefaced what he wrote as being his opinion. As to 1 Corinthians 6:9, this is Paul restating what is found in Leviticus 18:22 and is not just his opinion.
Heterosexuals who are professed Christians and engage in sexual immorality will be judged in accordance with God's morality, not their personal morality. Leviticus 20:13 condemns homosexuals to death and Levitisuc 20:10 condemns adulterers to death so the penalty for both immoral acts is the same - death.
Paul was (is) God's messenger and what Paul wrote was inspired by the Holy Spirit and those who choose to ignore God's truth as revealed through Moses, Jesus, Peter, Paul, James, and etc. have only themselves to blame for their eternal destination.