The Lord had destruction in mind for Sodom and Gomorrah before the angels personally visited.
Homosexuality was certainly rampant there, but, as one commentator mentions, homosexuality was rampant throughout that world and there was no violent overthrow from the Lord. “Going after strange flesh” evidently signifies something much worse than just homosexuality. It is a concept connected with like behavior by angels that caused their overthrow.
I would say that homosexuality combined with the attendant violence was the cause. S&G and the surrounding region were particularly affected by this behavior.
Exactly. It is the sin of Genesis 6.