Dogs have a simple mind and they always relate by their natural senses such as smell and touch. When you see a dog licking another dog’s private parts, it is actually a healthy and normal dog social behavior – just a kind of polite getting-acquainted through grooming and scent of one another. They do this whether they are sterilized or not.
When dogs meet for the first time, they will sniff and occasionally lick each other "private areas". This is how they get to know each other, it's actually a good sign that they are getting along.
Not only do dogs sniff and smell things out of curiosity, they taste and paw at things out of curiosity. Dogs learn a lot about each other by sniffing and licking the genitals of other dogs. They are curious how other dogs taste as compared to themselves.
Some of what they can discover about another dog via this type of investigation is: age, gender, over-all health, sexual readiness, status within pack and where the other dog has been in the last few days (what have they been ingesting).
Tasting is not uncommon, although most adult dogs will only tolerate a moment or two of this level of investigation. Younger dogs tend to linger longer, and typically when they do this to an older dog, the older dog will stop them after a moment, and this is how the younger dog learns the limit for such behavior.
There is nothing innately wrong with the behavior, but for the sake of etiquette and to keep the dogs from conducting such a lengthy check on another, you may want to disrupt it after about 10-15 seconds. Call the dogs to “come” to you and distract them with toys or games. Or interrupt the behavior with a firm "enough" or “no” command. If you do this consistently, they will start to learn that the behavior is only allowed for a few seconds and not minutes.