The thing you have to remember about Christianity is that there has never been a written set of rules which people had to agree to. The beliefs of the church(es) have always been a "work in progress", based partly on what the Bible says, partly on the practices of the early church, partly on how the church has developed and changed over time. Add to that the splits into different denominations, and it becomes very hard to say "all Christians believe X is wrong".
About the only things which all (most?) Christians have in common are the Nicene Creed link here and similarities of the wording of baptism, confirmation, and communion services. None of which gives a set of rules which you have to agree are the correct way to live your life.
The closest thing to a set of rules is the 10 commandments, which Jesus summarised as "Love God, and love your neighbour as yourself"
The 10 Commandments are -
I am the Lord your God: you shall have no other gods but me.
You shall not make for yourself any idol.
You shall not dishonour the name of the Lord your God.
Remember the Sabbath and keep it holy.
Honour your father and mother.
You shall not commit murder.
You shall not commit adultery.
You shall not steal.
You shall not be a false witness.
You shall not covet anything which belongs to your neighbour.
None of which mentions sexual behaviour at all.
So, although there are NT passages which are negative about homosexual acts, and all sex outside of marriage, the way you view the authority of the Bible affects how you think you should approach those passages. A Biblical fundamentalist approach says that every word of the Bible is there because God wanted it to be there, and therefore it has a huge amount of authority over how we should live our lives.
But a liberal Christian approach (of which I'm definitely an example) says that the Bible was written by humans as a response to their relationship with God, and is affected by the history and culture of the people writing it. So, wjhile it is very important, we should not be blindly ruled by it, but treat it as full of examples of what people have done before, and try to take the imporant and relevant parts of their example rather than assuming we must follow it exactly.