I'm a Christian, and I don't see that there is an issue unless it involves "lusting after" someone inappropriate. My understanding is that I can lust after my husband all I want. So, if that prompts my actions, or if it is strictly a physical reaction to physical stimulus, I don't see a problem with it.
Unless, of course, I am satisfying myself and avoiding my husband. I don't have a right to deprive him of sex. That's one of the problems I have with men and porn. Porn is cheap and easy and gets the payoff without any personal investment. (And it distorts a man's idea of what sex is, or what a real woman wants.) That makes it easy for a man to defraud his wife of a satisfying intimate relationship.
Men and women are capable of having orgasms in their sleep. I can't see a sin in something involuntary.