What makes a novel YA?

With the relocation of the Young Adult section from the back to the front of many bookstores, I find myself wondering what exactly it is that makes a book YA. I have been told by not a few people to not be surprised if a publisher wants to turn my novel YA. What does that mean exactly? Yes, my two main protagonists are under 18, but I wrote my book for adults. In researching the meaning of YA, I came across this on Wikipedia:

Young-adult fiction or young adult literature (often abbreviated as YA),[1][2] also juvenile fiction, is fiction written, published, or marketed to adolescents and young adults. The Young Adult Library Services (YALSA) of the American Library Association (ALA) defines a young adult as someone between the ages of twelve and eighteen. Authors and readers of young adult (YA) novels often define the genre as literature as traditionally written for ages ranging from twelve years up to the age of eighteen, while some publishers may market young adult literature to as low as age ten or as high as age twenty-five[3]. The terms young-adult novel, juvenile novel, young-adult book, etc. refer to the works in the YA category.

The subject matter and story lines of YA literature are typically consistent with the age and experience of the main character, but, beyond that, YA stories span the spectrum of fiction genres. Themes in YA stories often focus on the challenges of youth, sometimes referred to as problem novels or coming-of-age novels.[4] Writing styles of YA stories range widely, from the richness of literary style to the clarity and speed of the unobtrusive and free verse.

I can honestly say that this didn't tell me a whole lot. So is YA simply having young characters? Though the protagonists are under 18, the antagonist in my novel is very much an adult and rules by fear and domination, be it sexual, physical, or mental. Am I the only one that feels like this is inappropriate for young adult readers? I know I have many bloggy friends who write for the YA genre specifically and I would love to get your opinion. 

Is any topic appropriate for YA as long as the characters are young? Are you hurting yourself by insisting on publishing adult? Are morals really this lost in today's culture?