Hey mysterious ,
What we dream about is usually tied to what we think about while we're awake .
So a dream about sex , in that sense would be an extension of our preoccupation with sexual fantasy .
And in such case one should be on guard , for those fantasies not only can easily be wrong in themself , they often lead to sinful acts .
Because lust isn't satisfied with moderate behavior , that's why they call it lust .
If you are referring to what is called " a wet dream " , one that was not consciously induced from spending our waking hours indulging lust provoking fantasies ; then it may still be sin , but it's our flesh sinning , not our mind and spirit . As I believe the Apostle Paul may well have been referring to , at least in part , when he wrote :
Now then it is no more I that do it, but sin that dwelleth in me. <-----> Romans 7:17
For who is our " I " ? Is my body my " I " ? Am " I " my body ?
No , we are a bit of God's Spirit in a body , which makes us a soul .
Our soul is our individuality as one created to be in God's image .
That is our " I " .
So , sex dreams which we do not assist in the having , should not be defiling to the conscience , as would sexual fantasy which we pursued or failed to resist while awake . But they still evidence the selfish / sinfulness of our flesh , only not in a condemning way to our soul .
I hope you can understand what I'm referring to here .
Peace In The Beloved .
wm