Not really sure if this is ntr or not. She's in a kind of fantasy game that reads your desires and recreates them. All the people she's having sex with are part of the program. Apparently she got lonely because she saw kirito and leafa hugging. She ends up enjoying having the "kirito" watch her desperately while she has sex with other people because she feels like the real one isn't looking at her enough.
shiranai is actually correct. This has been translated by Doujin-moe and in the afterword the author states this isn't meant to be a bad end. Essentially Asuna's feeling jealous and wants Kirito's attention focused solely on her and after Kirito finds her masturbating he understand her jealousy and they awaken to a new play.