"That bit isn't considered, and if it was then actually Christians shouldn't even consider those who are gay to be sinful."
There are no quotes attributed to Jesus Christ about same-sex relationships - although he did strongly condemn heterosexual adultery.
To support anti-gay rhetoric Christians have to go to other sources.
1) The Old Testament Leviticus lays down a lot of proscriptions - including the one that is anti-gay. It is equally dogmatic that you should not: eat pork and shellfish; cut your hair or beard; wear mixed fibres. It also says you must: be circumcised; kill your children if they are disrespectful.
2) The New Testament letters by Saint Paul contain an anti-gay reference. He also says that women must be silent in church. The latter edict was responsible for the often fatal castration of thousands of young boys in the 18th century - originally to provide Catholic churches' choirs with a final small percentage of trained adult high voices.