Becky, the most thorough explanation I've seen comes from Harlequin's Love Inspired stuff - and there is now Steeple Hill Love Inspired, Romantic Suspence, "chick Lit," all sorts of stuff.
Characters should not be engaging in sex (OFF SCREEN sex, no on-screen sex!) unless they are married. HEre's some other information that may help you out with the "rules." This came from Harlequin's website, other Christian publishers may vary, but please note that some of these are the CBA rules. (Christian Book Asssociation.)
"There should be no explicit sex in these stories, and a minimum of sensuality and sexual desire. Both humor and drama have a place in these books; foul language, swearing and scenes containing violence do not. Though the stories may take place in urban environments, hanging out in bar settings, drinking alcohol or becoming involved in sexual situations is not appropriate for Christian characters.
Although the faith element is an important part of these stories, the degree of religiousness can vary. We would prefer that specific Christian denominations not be named, e.g., we prefer "Good Shepherd Christian Church" to "Good Shepherd Baptist (or Presbyterian, Methodist, etc.) Church." Though the heroine can be conflicted about being "cool" or finding her place in the contemporary secular world while maintaining her Christian values, her behavior cannot stray from acceptable CBA norms. Because Steeple Hill Books sells to both CBA and ABA bookstores, we must adhere to CBA conventions. The stories may not include alcohol consumption by Christian characters, dancing, card playing, gambling or games of chance (including raffles), explicit scatological terms, hero and heroine remaining overnight together alone, Halloween celebrations or magic or the mention of intimate body parts. Lying is also problematical in the CBA market and characters who are Christian should not lie or deceive others. Possibly there could be exceptional circumstances (matters of life and death), but this has to be okayed by an editor. "
http://www.eharlequin.com/cms/learntowrite/ltwArticle.jhtml?pageID=040301wu01001
Hope that helps!
Susan G.