Why do male dogs that are house trained urinate in the house?

Some indoor urination in dogs can be marking--thus territorial--behavior, but this is most common in particularly territorial dogs, adolescent males (or more territorial females), and dogs with behavioral problems. Most dogs can easily be taught not to urinate indoors.

To a dog, the house--aside of the dog's sleeping area--is just an extension of the outside world unless it is taught otherwise. Most don't understand, without training, that it is inappropriate to relieve oneself in the living room. After all, that's not a place where he has to sleep and, even if he does sleep in the living room, he can just pick a place he hasn't urinated.

Dogs are intelligent and trainable creatures, however, and once they understand that it is proper pack etiquette for the 'pack' that is your family for him to urinate outside, barring other problems, you'll generally have a housebroken dog. Difficulty in housebreaking varies greatly by both breed and individual dog.