To clarify, no where, either here or in the forum, has anyone said Christians are obligated to write anything, theme-wise or otherwise.
All I've ever said is Christian writing of any sort is one where the author is submitted to God, as the Church teaches we should be, and has written what He told them (lead them) to write. The material might be wholly secular if judged by the world's measures rather than the biblical measure of the author's relationship with Christ.
As I've said, all books, whether Christian or Non-Christian, have a message and can be measured for Truth by the scripture apart from any regards for genre, personal preferences, or target audience. But that is a whole different topic.
Again, what makes your work Christian or not has nothing to do with the words on the page in and of themselves, and everything to do with your relationship to God and submission to Him in this area of your life. If anyone here really, truly doesn't want to Christ to be Lord over your writing and hence are convinced He doesn't care--you're free to make that choice, whoever you are. I do not say this to condemn you. We all have areas that we struggle to lay down control of to God, but still we remain His. I do pray that someday you will become willing to lay this crown down at His feet, but that is still up to you.
Though, one chief barrier is we tend to fear He'll demand the writer's version of being a foreign missionary, which is why stuff no one here is saying keeps getting refuted. In reality, He's probably already positioned you where He wants you. He just wants you to scoot over and let Him at the keyboard; or for Country music fans, to let Jesus take the wheel.
But, I've said more than enough. Please everyone, debate this with Him. I'm finished.