Anatomy of penis in dogs is different from humans.Penis of a dog doesn’t have glans ( tip of the penis with sensitive tissue) instead of that it has bulbus glandis at base of penis shaft.Penis in dogs has a long narrow bone: Os penis.Therefore dogs don’t need erection for intercourse like human.
Instinctively male dogs have affinity for opposite sex.Even after fixation sometimes they show affinity for female dogs with behavior like erection with hump.It takes time to forget this feeling.This duration depends on when your dog is neutered ( fixed). Earlier neutering decreases this duration.
Recommended age for fixation in male dogs is 6 months old.