Never had dreams like this, but i cant explain it to me... think theres some emotions you supress that show up in your dreams.
imo it has to do with the schemata.
You get horny in your dreams, combine that with something you see in your dream.

Lets say :

You see a bus. You get horny, you combine that with a person you saw in such a bus. you live your hornyness with that person. cake

( but its pretty weird and psycho if you get horny by seeing a bus hahaha)