Ok. So the first thing that happened to my brain was a short-circuit of my limbic system.
The second thing was my obsessive-compulsive personality traits kicking in and me fixating on how Darth Vader and Voldemort come from two different universes, that the timelines of the two stories could in no way collide unless we broke the fourth wall which made me remember how I wasn't able to let go of a scene in Deadpool which was maybe the only one they wanted to be logical, but they messed it up and it deeply marked and traumatized me.
Now, here comes the third thing. The problem with sexuality is that it is not something fixed. It is fluid (I am not talking about sexual orientations or even asexuality here). It might indeed be very strange to imagine a character we considered as detached from sexuality to engage in sexual actions. However, what are the motivations behind it. Sex can be used as leverage. Did the characters (Darth Vader and Voldemort) submit to a sexual act to reinforce an alliance? To submit the other one? To cause shame?
Generally, in literature and art, sex is about everything but sex. Power, revenge, impulsivity, name it!