If you check standard moral theologies (e.g., Henry Davis’s excellent four-volume Moral And Pastoral Theology–long out of print but available by used book services), they will point out that things that you do in dreams aren’t sinful, though it can be sinful to do things while awake in an attempt to cause dreams with sinful content.
These don’t really address the question of lucid dreaming directly, though, since they are talking about ordinary dreams.
For those not aware of the distinction, lucid dreams are those in which you are aware that you are dreaming. Sometimes people who are having a lucid dream are also able to take control of the dream and cause things to happen the way they want them to. It’s even possible to train yourself (while awake) to have lucid dreams while you sleep, though not in a reliable fashion. (I.e., it’s possible to train yourself to have lucid dreams more often, but you can’t guarantee that you’ll have one on any given night.)
Though the standard moral theologies don’t address the question of lucid dreams specifically, the basic answer holds: You are not sinning (certainly not more than venially) no matter what you do in a lucid dream.
The reason is that you still lack the use of reason and are thus incapable of committing the fully human act needed for mortal sin. You usually lack sufficient use of reason just before you fall asleep and just after you wake up–unless you are jolted back into the waking world for some reason. It normally takes your brain at least a few seconds to spool up your FTL reason drive.
The condition of a person having a lucid dream is in many respects simliar to that of a person who is drunk: Drunks thinks that they have more control than they do. That’s why they get behind the wheels of cars and go driving when in fact they are totally unsafe drivers at the moment.
They even do have a measure of control–they can drive, or try to drive, the car wherever they want. They’re just don’t have enough control to drive in a responsible manner, even though they think that they do because their reason has been impaired and it keeps them from recognizing this.
It’s the same with lucid dreamers. They feel like they are in control of their actions, and they do have a measure of control of the dream. They can drive, or try to drive, the dream wherever they want it to go. They just don’t have enough control over themselves to dream in a manner that triggers full human responsibility. They may think they do, but they don’t, because while they’re asleep their reason has been impaired, whether they recognize this or not.
Lucid dreamers, like dreamers in general, simply do not have the level of reason needed to perform fully human acts, and so they are not capable of committing mortal sin.
They may feel in control. They may even have moral-theological debates with themselves in the dream about whether it is right or wrong to undertake a particular course of action. But they lack the reason necessary for mortal sin.
Period.
So don’t worry about that.
On the other hand, don’t go training yourself to do lucid dreaming if you intend to use it as a means of engaging in sinful fantasy behavior. That would count as doing something while you’re awake to deliberately bring on sinful dream content.
