Jesus never spoke against LGBTQ, so why is it a sin?

Thanks for your question. Jesus did speak against LGBTQ. He affirmed the Genesis account of marriage as being between one woman and one man (Matt 19:4-6; Mark 10:6-9). Such a marriage - biblically the only legit marriage - excludes all LGBTQ relationships.

Jesus also spoke against the sin of “porneia” which condemns every sexual sin -including LGBTQ - under the sun ((Mark 7:21).  

Since the Bible is God’s Word and it speaks against homosexuality and Jesus is God then Jesus did speak against LGBTQ (Gen. 9:20-27; 19:4-11; Lev. 18:22; 20:13; Judg. 19:22-25; Ezek. 16:50; Rom. 1:26-27; 1 Cor. 6:9; 1 Tim. 1:10 and possibly Ezek. 18:12; 33:26; 2 Pet. 2:7; Jude 1:7; Rev. 21:8; 22:15). We could also add the texts referring to homosexual cult prostitution (Deut. 23:17-18; 1 Kings 14:24; 15:12; 22:46; 2 Kings 23:7; Job 36:14). 

There is no such thing as a LGBTQ Christian. You are either LGBTQ or a Christian, but you can’t be both at one and the same time. Scripture is very clear saying in 1 Cor. 6:9-10, “Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God.”  

The good news is that LGBTQ is not the unpardonable sin. Though LGBTQ can and will damn someone to hell it may also be forgiven, just like adultery, thievery, idolatry, or covetousness, et. al. Everything begins with Christ. Know him first and he will teach you how to live life.