There is a difference between urine marking and litter box aversion. With litterbox aversion, the cat does not want to use the litterbox for various reasons. Urine marking is considered to be a territorial behavior, or in some cases stress related. Both male and female can mark, but it is more common in the male. Marking may be increased in the presence of a female (for the male) and in multiple cat households.

Marking is different than not using the litter pan. With marking, the cat will back up to an object, usually vertical, raise its tail and leave small amounts of urine. Although the urine is enough to spread down the object, it is usually not a large puddle like an actual urination. The marking is often in areas near doors or areas of high traffic. Anything that makes the cat believe his territory has been invaded can trigger a cat to start marking.

If there are a large number of cats in the yard and your cat can see them, this will often cause marking to begin as he feels the need to protect his territory from these potential invaders. Or if there are multiple cats in the house together, one cat may feel the need to assert his presence and mark. If there is tension between the cats in the household, either the aggressive or the passive cat may mark.

If you are unsure which cat is marking, it may be necessary to separate the cats to determine who is the culprit. If separating the cats causes the marking to stop, it is likely due to dynamics between the cats in the household. If you still cannot tell which cat is the culprit, speak to your veterinarian as there are ways to change the cat's urine to tell who it is.
If the marking is associated with stray cats outside, it is advisable to limit or prevent your inside cat from seeing them. You can do this by preventing your cat from getting in the windows and blocking off any access to doors with glass that allows the cat to see outside. You can make your yard inhospitable to cats by installing motion sensitive sprinklers and not leaving food outside.
To treat marking, there are several options. Studies have shown that the use of pheromone products, such as Feliway, will decrease marking by making the cat feel he has marked due to the scent the cat can smell. These pheromone products are able to be detected by the cat's sense of smell which is many times better than ours but does not give off any scent that humans can smell.

If altering the environment does not work, there are medications that can be used to change the cat's desire to mark. These must be prescribed by your veterinarian and will usually require blood testing prior to use.

Remember, marking is a social issue, not a sexual issue. If it is caused by the number of cats in the house, it may be necessary to decrease the number of cats in the household. Consult your veterinarian to determine the likely causes of the problem and develop a treatment plan. Be prepared to answer questions about the dynamics of the household and interactions between the cats as well as interactions with humans. The more information you can give your doctor the better he/she will be able to treat your pet.