Leviticus 20:13. No matter how it is interpreted by modern Jews and Christians, the passage is quite clear as to what the Bible states about Homosexuals. Put them to death.
An outsider can only question that if modern Christians feel they can reject so many tenets laid out in Leviticus, which includes putting people to death for a number of things or declaring some other activities to be punished with infertility (biologicaly false these days) why is it that so many retain that homosexuality is a sin when it is taken from the same book of other iniquities they ignore and state no longer applies since Jesus.
I've long believed that bias towards homosexuality has become an ingrained cultural belief beyond religion. I've known far too many people who did not like homosexuals but were not religious in any sense. I think archaic Biblical passages are used in a case of confirmation bias.