If your understanding of the Bible doesn't match reality, you're probably reading it wrong.
So, you think you believe what the Bible says? How about this verse?
5 Behold, I was brought forth in iniquity,
And in sin my mother conceived me. - Psalm 51:5
Or this one?
9 What then? Are we better than they? Not at all; for we have already charged that both Jews and Greeks are all under sin; 10 as it is written,
“There is none righteous, not even one;
11 There is none who understands,
There is none who seeks for God;
12 All have turned aside, together they have become useless;
There is none who does good,
There is not even one.”
13 “Their throat is an open grave,
With their tongues they keep deceiving,”
“The poison of asps is under their lips”;
14 “Whose mouth is full of cursing and bitterness”;
15 “Their feet are swift to shed blood,
16 Destruction and misery are in their paths,
17 And the path of peace they have not known.”
18 “There is no fear of God before their eyes.” -Romans 3:18
These are part of the founding principles of the Reformation.
Sola Scriptura. We are ALL born sinners, slaves to sin, ever since the Fall, and yet we are responsible for our lives. Why in the world would you be surprised if some people are born gay?
Because you don't believe the Bible, that's why.