The premise is based on the recent arc of Railgun which has yet to finish because it is being dragged to hell and back. Basically, the card is called an Indian Poker card, which allows the person who uses it to experience the experiences of the person whose "experience" is inside the card. Hence why Saten is shown here with various stuff since she uses the cards to learn new skills. Basically, here, Misaka apparently uses a card of a person that Touma is fucking, which according to this doujin seems to be Misaka herself because the reflections of the person whose experiences are in that card remain instead of the person who is using it.
In the end Misaka relives a moment where she is fucking Touma, and her experience somehow ended up in an Indian Poker card to be sold on the streets.