A non-mormon friend of mine in college (USU) asked me about the early church teaching that god had sex with Mary. I had never heard of it before, so I started researching early teachings of Smith and Young. That got me out pretty quick!
I believe this doctine was in Doctrines of Salvation, which used to be in the paperback church history set, but I threw all this stuff out a long time ago. If any of you see Bill Marr's "Religulous" which is very funny in parts and right on the line between scary, funny, and creepy in others, some exmos talk about this teaching of Elohim having sex with Mary. The explanation was that Matthew and Luke say that no "man" knew Mary, but Elohim was a god, so the idea was that Matthew and Luke are being intentionally cagey in their accounts. (Actually, there are a lot of these kinds of word games in mormonism it seems.) Anyway, Marr shows a clip from some anti-mormon film (I am guessing) engaged in some pretty intimate naked foreplay with Mary (well, wouldn't Elohim have some foreplay?). Of course, this doctrine is kind of buried under all the temple night and chapel cleaning rosters and the need to shovel that tithing into the coffers, but it used to be easy to find if you know where to look.
For me, it was simply realizing that I didn't believe in the spiritual/religious aspects of it.
I remember at other times trying to rationalize that I could recognize how certain temporal aspects of the church were things I could agree with (I can agree with a Law of Chastity, to an extent, and will a Word of Wisdom)...but I hated the spiritual parts.
But one day, I realized, "Wait a minute...the entire point of religion is the spirituality...I can do all of these nonspiritual things without pretending to buy into the spiritual stuff." Then of course, over time, various events made me dislike temporal aspects of the church less (hard to like the Law of Chastity, Proclamation on the Family, etc., as much when it is used in Proposition 8 campaigning)
My spark happened on my mission. I began to really analyze the BOM and kept finding incongruities. I study evolutionary biology in college, but that only made me question how active God was in our lives. The old testament was the catalyst for me. I just couldn't believe in a god like that. I researched lots of types of God, hindu, buddha, thor; but my doubt gradually spread to all types of super naturalist belief.
I never really had a testimony, and spent most of my time in church goofing around. Due to family differences, I only had to go to church every other week, so I wasn't really involved in the ward community. By the start of Young Womens I was openly atheist, so "they" didn't hassle me about going to church :)
I guess the spark started as soon as I could comprehend that I was a woman, and as such I was in a lower position within the church. That, and all the "god" silliness.
Looking at several of the other posts on this thread, it looks like I was in much deeper than most. I was BIC. I am the descendant of polygamists in the ancestry of both my parents, including a noted colonizer of northern Utah/southeast Idaho. Church was my social world as a teen, which I do not really regret. I graduated from BYU, served a mission, read all scriptures cover-to-cover, married a member in a temple, taught GD classes, had too many children, the whole package.
Several fire starters:
* Never really got the "testimony" despite having emotional experiences
* Very dysfunctional LDS marriages for both my older sisters inclduing abuse and ending in divorce. both dude had personality disorders, one is bi-polar.
* Increasing distance by church leaders from pursuing evidence-based truth about the universe and particularly about the BOM and PoGP.
* Increasing realization that church management and expansion is more important than happiness of the members.
* A bout of depression
* Fathering children who have turned out to be "difficult" personalities with ADD and one with autism.
* Understanding that there is no place in The Kingdom (or explanation) for autistic individuals (in contrast to Down's syndrome).
* Realizing that the Ward we were in was offering us nothing socially, emotionally, or materially.
I never really knew there was another choice. I just totally believed the Joseph Smith story. Then me and my wife really dedicated ourselves to the church to get temple worthy, so we could be sealed. Well I guess we got to deep and learned to much. We read the BoM together and was blown away at the silliness and crazyness of it. I used to defend Mormonism and the BoM. Now I have actually read it I feel like such a stupid ass for defending that obviously made up book. I still believed, just not Mormonism. Well then I read Hitchens 'god is not great' and that pushed me to full blown non-belief of the supernatural. My wife was indoctrinated pretty hard from her earliest memory. So it took her a little longer and alot more tears to get away from the church.