Prior to Paul, the only Jewish objection to homosexual behavior was penetration and ritual prostitution:
[Lev 18:22 NKJV] (22) 'You shall not lie with a male as with a woman. It is an abomination.
[Lev 20:5-6 NKJV] (5) 'then I will set My face against that man and against his family; and I will cut him off from his people, and all who prostitute themselves with him to commit harlotry with Molech. (6) 'And the person who turns to mediums and familiar spirits, to prostitute himself with them, I will set My face against that person and cut him off from his people.
But Paul objects to homosexuality on the basis of what he perceives as unnatural:
[Rom 1:18-32 NLT] (18) But God shows his anger from heaven against all sinful, wicked people who suppress the truth by their wickedness. (19) They know the truth about God because he has made it obvious to them. (20) For ever since the world was created, people have seen the earth and sky. Through everything God made, they can clearly see his invisible qualities--his eternal power and divine nature. So they have no excuse for not knowing God. (21) Yes, they knew God, but they wouldn't worship him as God or even give him thanks. And they began to think up foolish ideas of what God was like. As a result, their minds became dark and confused. (22) Claiming to be wise, they instead became utter fools. (23) And instead of worshiping the glorious, ever-living God, they worshiped idols made to look like mere people and birds and animals and reptiles. (24) So God abandoned them to do whatever shameful things their hearts desired. As a result, they did vile and degrading things with each other's bodies. (25) They traded the truth about God for a lie. So they worshiped and served the things God created instead of the Creator himself, who is worthy of eternal praise! Amen. (26) That is why God abandoned them to their shameful desires. Even the women turned against the natural way to have sex and instead indulged in sex with each other. (27) And the men, instead of having normal sexual relations with women, burned with lust for each other. Men did shameful things with other men, and as a result of this sin, they suffered within themselves the penalty they deserved. (28) Since they thought it foolish to acknowledge God, he abandoned them to their foolish thinking and let them do things that should never be done. (29) Their lives became full of every kind of wickedness, sin, greed, hate, envy, murder, quarreling, deception, malicious behavior, and gossip. (30) They are backstabbers, haters of God, insolent, proud, and boastful. They invent new ways of sinning, and they disobey their parents. (31) They refuse to understand, break their promises, are heartless, and have no mercy. (32) They know God's justice requires that those who do these things deserve to die, yet they do them anyway. Worse yet, they encourage others to do them, too.
Is Paul opening the door to humanism by inviting us to ignore Torah and inspired writings and instead look at the stars to identify right and wrong?
He claims no basis in Torah and no basis in tradition for his animosity toward homosexual behavior, only his sense that it is unnatural, as revealed in the stars and such.
And he lays this premise as the first foundation stone he lays in his Romans treatise. Men are bad because they are homosexual as the stars declare. That seems to be his premise for all that follows.
So was Paul ultimately waxing philosophical rather than expounding Torah or any explicit teaching from the Messiah (since Jesus never mentioned homosexual anything)?
This is, at the end of the day, a question of both hermeneutics and epistemology. Do we allow an apostle to appeal to nature to dictate dogma? Or is Paul beginning a new, Hellenistic humanism where our ethics arise from nature?