The reason why animals get a erection while urinating is nothing to do with sex. I've seen it in dogs, cats, horses (regularly), and big cats. It's simply because the nerves which control vasorestriction - the narrowing of the pudendal veins that brings on erection - branch off the same nerve bundle that controls the urinary sphincter (valve), the genitals, and the anus. I once saw a male jaguar with a full erection which defecating.