Christians considering homosexuality sinful as a whole may be a very modern thing. Before that, versions of the Bible were all talking about pederasty instead:
Has 'Homosexual' Always Been in the Bible?
Translations have always been updated due to the constant changes in language throughout the centuries. For example, if you read the old English Bibles you may not be able to read them well due to the old spellings, grammar, and word selections. Even the KJV uses words that mean entirely different things today then they did back then. Case and point: the word "unicorn" appears in all old English Bibles. Skeptics have used the unicorn in attempts to drive in their point that the Bible is just a fairytale. However, what unicorn means today and what it meant in the 16th century are two entirely different things. The original Webster's Dictionary is used today as a lexicon for the English of the KJV.
Unicorn
http://webstersdictionary1828.com/Dictionary/Unicorn
There you can see that what unicorn meant back then doesn't mean that today.
Thus, revisions in translations is an ongoing process.
As for homosexual being immoral in the Bible, a good look throughout history makes an easy case for homosexuality being a sin. But even if we ignore all the key verses that use the word homosexual today, there is still the Romans 1:18-32 which speaks about God's wrath in the last days being poured out on evolutionists and the motives that drive their disbelief.
Romans 1:18-32
https://www.biblegateway.com/passage/?search=Romans+1:18-32&version=MEV
Regardless of what translation you use or in what language it provides a description of the act and not just a word. Hence the word homosexuality never appears in this passage and yet what is described is very clearly homosexual.
In the Old Testament, Homosexuals wanted to have sex with angels due to their perverse nature. God destroyed Sodom and Gomorrah because of such wickedness.
Back during Obama, I remember how the Supreme Court passed gay marriage. And what do homosexuals do with their newly found right? They use it as an excuse to attack Christians and try to pervert their children.
A Message From the Gay Community" Performed by the San Francisco Gay Men's Chorus
So as you can see, the prophecy in Romans 1:18-32 is proven true through the perverted nature of homosexuals who want to have sex with children and corrupt their nature. And what did Jesus say about all this?
Matthew 18:1-6
https://www.biblegateway.com/passage/?search=Matthew+18:1-6&version=NKJV
So its not homophobia that causes Christians to reject homosexuality. Its nature. Its the nature God created for us to live our happiest and most productive lives. Homosexuality is contrary to human nature. The very act itself mocks God. So homosexuality is a sin that, like all sins, must be repented before entering the kingdom of heaven.