It's already way past bedtime but this post really caught my eye. On one level I can agree with you but, of course, it's all too easy to run into the predetermination cycle again before long!
You say: "I believe He designed human sexuality (and the high-level blueprint for sexual development), but that He did not specifically craft each and every nuance of a person's sexuality."
and I suppose, going on from that, that it's possible to show how problems and crossed wires in early relationships might be put down to responsible people's wrong choices (neglect etc). But then you run into the other fact (and this is partially relevant to Clolibre perhaps) that numerous amazing artists (say,Schubert, or van Gogh to mention but two) have taken their inspiration (their need to create) from psychological problems stemming from childhood. (Weakness which unexpectedly produces great beauty and insight).
These problems encountered cannot easily be seen as a mere oversight by God and need to be understood as a part of his overall 'plan' somehow and it is perhaps that all-encompassing vision of the presence of God in our world that we are actively seeking through spiritual relationship with Him . . .
Although I can see that my sexual crises link with other problems that I have with relationships which God might certainly wish me to overcome (only possible, I am sure, with help from others - which may not be forthcoming for heaps of reasons) I am also aware that my tendency to isolation is also a fundamental aspect of my relationship with God . . . God has a way of turning evil to good in uncanny and unexpected ways. Humanity is nothing if not a problem-solver, but when the problem is too difficult he is also designed to get on as best he can with crutches if need be . . ."the journey's the thing"...
and now to bed!!