:sigh:
It matters because of the hypocrisy of those Christians who insist on imposing the parts of the Bible they like on the rest of us, while hypocritically ignoring the parts that don't suit them.
Equating homosexuality, not with fornication or gossip, but with murder carries with it an implicit condonation of violence.
***
As for your question about why the Ancient World didn't have a concept of gayness equivalent to ours, it's because they didn't have a concept of romantic marriage equivalent to ours. Marriage was an economic institution that was the duty of everyone. Children were an essential addition to the family labor force, and also the only security for one's old age, as they are in all agricultural societies.
There are allusions in Greek and Japanese sources to what we would call adult, gay relationships, but they are few. Almost everyone married and tried to have children, regardless of his or her personal orientation. It was a matter of survival, personal and for society.
Modern concepts of homosexuals arise with the Industrial Revolution, because for the first time, one worker could support himself with his labor. Before that, you needed an entire family working to avoid starvation.