Leviticus is one of the books of the Bible accepted as canonical by the Catholic and Orthodox Churches.
For sexual immorality, have you not read Genesis 19?
4 Before they went to bed, the townsmen of Sodom, both young and old — all the people to the last man — surrounded the house. 5 They called to Lot and said to him, “Where are the men who came to your house tonight? Bring them out to us that we may have sexual relations with them.” 6 Lot went out to meet them at the entrance. When he had shut the door behind him, 7 he said, “I beg you, my brothers, do not do this wicked thing! …
And Matthew 5:
27 “You have heard that it was said, ‘You shall not commit adultery.’ 28 But I say to you, everyone who looks at a woman with lust has already committed adultery with her in his heart.
And Matthew 15:
19 For from the heart come evil thoughts, murder, adultery, unchastity, theft, false witness, blasphemy. 20 These are what defile a person, but to eat with unwashed hands does not defile.”
I don’t understand how anyone would think of the book of Leviticus as being ridiculous. The author(s) are attempting to maintain a cohesive society based on ritual, derived from the Mosaic tradition. The mixed fabrics and boiling young goats in their mother’s milk, sodomy, and such were meant to set the Israelites apart from their pagan neighbours.
The Canaanite temples had male prostitutes, so the prohibition relates directly to those religious practices. But indirectly, it comes from the moral law concerning legitimate sexual relations. Homosexual is not the same as a “sodomite.”
People realize, yes, that Leviticus is a Priestly document, and embedded within it is the Holiness Code. It is an extraordinary manual of ritual and orthopraxis, much of which Christianity took as a guide for its own priesthood.
The Church may teach that slavery is not inherently wrong. The book of Leviticus may have God himself saying it’s perfectly fine to take slaves. My standard of morality says slavery is inherently wrong.