I used to have sexual dreams about my mother's ex boyfriend, though this was while he was living with us not recently. I've learned that a lot of his behavior towards my sister and I was a lot more sexual in nature than I had been willing to admit (and still not sure how much of it I really admit, at least on my end, it was a lot more overt with my sis). I feel as though my brain was extrapolating from the cues I was getting, even though he never actually touched me.

It's hard to say whether your dreams are based on repressed memories or not, but from your description your stepfather's behavior was very inappropriate and likely sexually motivated even in the absence of an obvious sexual act, your mind may be building your dreams from those experiences.