As for many of us, my believe in God was severely challenged after my hormones came into play (pun not intended). There is only so many desires you can suppress in a child, only so many guilt-feelings for having wasted seed.
So the question came up in my mind: WTF did God have in mind when he forbade us to masturbate on the one hand, but created hot chicks on the other hand? And let our bodies react to it automatically as a knee-jerk reaction (pun again not intended). We are talking about urges that I, like 98% of the male human species, could not control. So: either God is a sadist or masturbation is not wrong.
It seemed to be the former, at first. The Kitzur, chapter 151 (notice that I try to avoid a word that sounds like semen), says that spilling seed is the worst aveirah (sin) you can commit and its punishment is Misah Bidei Shamayim (death by the hand of Heaven). The The Zohar says that there is no teshuvah (repentance) for this sin and that all your spilled semen is murder, every single sperm is bloodshed.
Later on in life, I discovered that some other boys in yeshivah struggled with it as well. One day in the dorm I discovered a notebook of a friend of mine that contained x-rated stories that scared the hell out of me.
Shidduchim did not make things better: After years of fruitless searching, with no end in sight, I was so frustrated that I made my first trip to a brothel. Many trips were to follow (inasmuch as I was able to afford it at that time). I was and I am still ashamed of this, could hardly live with the contradictions: spend an hour in the evening with an anonymous Candy, Betty or Janine, after which I went home and said Shema before going to sleep, to combat the mazikin (demons) I thought I had created. Again, either God was a sadist or masturbation was something natural.
It took me some time until I was ready to question my faith. In fact, it would still take some years. But, back then, the seeds of doubt were already sown.