Remnants of an addiction, the fact is, your brain just reminded you of your habits. I've had many dreams, more than 10 so far in my span of rebooting. To me it occurs usually when sexual activity is suppressed, I've never had them when I've been actively fantasizing or had PMO. Forget about it, it's not healthy to overthink those dreams, usually it just a reminder of how good it can be and then you start thinking, trying to reconstruct how it felt, looked etc. etc. Interesting, just like at some point in rebooting off addiction it felt interesting to go check another video or start day dreaming about something "interesting" when feeling bad, guilty in this case. It's just another gap through which the addiction attempts to resurface, I believe pathways are being reinforced as this experience can be VERY shocking.
So far, almost in half of these dreams I've realized I have so say no to "that" and the next day I wake up feeling relieved when I remember that I controlled myself even while sleeping. Just understand that this can happen and no matter what, at any cost you cannot give up, eventually it sinks in and you start to realize it in dreams. Certain techniques are possible to create a more formidable defense against it just for the sake of interest, but if you are interested, I can give you an example on how to condition against it more effectively, though I personally find it a bit extreme.
I can also give you a very unique example of how once my brain literally tricked me into orgasm/porn in dreams after I already said NO once in it. I hadn't slipped a sexual idea in my mind for more than 10 days.
Basically a long time ago in my one of my first somewhat successful attempts of a reboot I had a dream just like any other, the first part was that I was on a yacht/boat enjoying my time steering it, enjoying the view and so on. The next moment I find myself in a room on the yacht that has a desk, just a blank desk with a chair and for some unknown reason I was very tempted to go sit down and "relax" (we all know this feeling) and I got suspicious as I recognized this temptation, I started to shake my head in the dream without saying a word, feeling disgust, basically a no. Then later on in the same dream I was sitting and pressing against a brick wall in a street simply contemplating the surroundings I very well know, doing absolutely nothing, then the next moment some people came and handcuffed me and I felt that this was expected, I felt guilty as I thought I did something very bad, then a feeling of compliance followed when they were taking me away. The dream quickly changed into a new set and I was walking down white/gray hallway with dim lighting with many rooms, nobody was around, but I felt as if I was obligated to go inside one of them. I entered one of the room and there were 2 other people sitting in a "class" type of room with many desks and chairs, I sat down near one of them, I looked out of the window to the left of me which had a relaxing sight, the next moment I turned my sight in front of me, a monitor was already there showing me some fantasy/pornography. I was so shocked that my view completely transition from the room into the fantasy, the most intense sexual dream I've ever had and I saw even some "new" things I've not thought before, obviously I had very intense orgasm.
It's truly amazing how your mind finds gaps for the conditioning you've built over time through different states of consciousness to maintain its status quo, treat it as a formidable opponent, because your mind has a mind of its own - subconsciousness. When you are fighting this addiction, you are fighting with yourself. I was so impressed by this dream that after it I started to call all techniques of rebooting - strategies, because that was so tactical.
The sexual content is not necessary on your screen, the addiction is much more than just blunt viewing of explicit material. Every urge starts with a state of consciousness of either being anxious, lonely, nervous, angry, restless, a sense of losing something important or a mix of all kinds of feelings to push yourself to idea that is going to give the necessary relief at the moment you are craving it the most. Stop the state of consciousness, stop the addiction. That's why activities are so important during rebooting, it takes your mind off of these feelings.