Sasuke only attacked Orochimaru when he was in his deathbed during the last day of his current body...Even Sasuke himself admitted he only beat Orochimaru because he was weakened. Are you going to call Sasuke a liar, OP? Orochimaru can beat a LOT of people, including Kakashi, Deidara, Might Guy, Jugo, and Suigetsu.
How is Sasuke not gay? He has turned down/tried to kill every female who has hit on him. Would you turn down Sakura, Karin, and Mizukage...Then consider yourself straight? Have you ever seen him kiss a female or even show ANY interest in the other gender?
Orochimaru is coming back because no Uchiha has been able to permanently deal with him...Sasuke, Tobi, and Itachi all tried to end him but they all failed. Kabuto will have to take care of what the Uchiha could not.