Licking each others’ private areas is actually a normal dog behavior. Dogs use their noses and mouths to interact with everything.
Assuming your dog is well-socialized and healthy, licking the other dog’s private areas is normal. It’s a dog’s way of getting acquainted with the other, to show him some friendly affection, or simply due to curiosity.
It has nothing to do with being a male or a female - it’s not sexual in any way.
Dogs are drawn to those areas because they contain a high amount of pheromone-producing apocrine glands. Those give the dog a unique smell (and taste), which other dogs may find interesting.
These pheromones give away many pieces of information about the other dog, such as age, sex, mood, and sexual readiness. The unique scent can also help a dog recognize and remember another dog he met in the past.
Usually, dogs that are uncomfortable with the behavior will stop it themselves. Unless the behavior becomes obsessive, there is no reason to do anything about it.
While it’s normal if done briefly, obsessive licking may indicate that something is off.
Sometimes a dog will lick another’s private areas because of some health-related issue around the licked dog’s private areas. The licked dog could be trying to clean an injury, infection, or some leftover urine in that area.
It may be worth having a vet check out the licked dog in that case.
Apart from medical issues, a lack of proper socialization could also be the reason. The dog obsessively licking may simply not know any other socially acceptable way to get acquainted with the other dog.
Typically, you want to let your dog just be a dog. Preventing your dog from engaging in usual dog behaviors, like the licking behavior, may cause behavioral issues in the future.
If the behavior is obsessive or the two dogs don’t seem at ease while the licking is happening, you can interrupt the behavior.
Let your dog lick (or get licked), and if the behavior doesn’t stop within a few seconds, call your dog back to you and redirect his attention elsewhere. Over time your dog will learn that doing this for too long is not acceptable.
Neutering/spaying your dog may also reduce this behavior.