This is something that I'm genuinely wondering about. I know that I have evil desires because I'm a fallen human, but what causes them? Somewhat ironically, this is what we're discussing in my Christian Philosophy class at this time, but I still haven't received a satisfactory answer. Saying, "It's the way I was brought up" doesn't help too much.
So what makes me more vulnerable to homosexual desires than, say, violent impulses?
Not to minimize your pain, but G-d often doesn't talk to us about why. He talks to us about our choices given the reality that we face. Why did the Jews face the Holocaust? G-d may never say. But He was very clear on what He expected His people to do about it, and thanks be to Him good Christians and Jews did what they could to help out those who would be doomed - often at the risk of their own lives.
I must admit that sometimes it frustrates the h - er - pants off me. But the Bible is much more clear on how we are to behave than it is on why things are the way they are. Job cried out, "Why?" and G-d replied, "Who is it that darkens my door without knowledge?"
Why do some men want to kill others? It's not about the why, it's about the commandment: "Thou shalt do no murder."
Why do some men want to sleep with other men? It's not about the why, it's about the statement, "For a man to lie with a man as with a woman is an abomination."
Why has G-d allowed men to be born in a fallen state for well over 6,000 years when He could just create the new Heaven and new earth today and be done with it? Why did He choose to allow His own Son to die to redeem us rather than simply casting us into the pit as we deserve?
He doesn't say why. He just says, "Come unto me, all who are weary and heavy burdened, and I will give you rest."
Shalom.
Everyone is individual, like snowflakes. We each have our particular failures and weaknesses, and strengths.
There are many ways to look at it; those who have been forgiven much (and repented much first!) often love much.
I'm no expert on original sin but, That's what it is. "Where do evil desires come from," is the wrong question. Our desires are inherently evil, selfish, etc. It is the heart (the desirer) which must change or, more precisely, be exchanged. After salvation, the flesh still tries to decieve the spirit...and so on.