Human Replica Droids were complex droids designed to be indistinguishable from humans. They were covered in synthskin or sometimes cloned human skin. They were pioneered by Project Decoy, whose main goal was to create espionage droids to infiltrate enemy bases. Why, the idea you could have sex with them never even entered their heads. Nope. Espionage. That's the ticket.
Female HRDs were warm, squishy, and accommodating. Male HRDs were hard, eager, and never impotent (unless they were programmed to be, in which case they hated their programmers).
In some cases, a Human Replica Droid was programmed to believe it was a real human, complete with false childhood memories. They didn't react well when they get cut open and saw their circuits. They often started muttering about attack ships on fire and glittering C-beams.