True humping, in an attempt to procreate, will usually only occur in dogs or cats who have not been fixed. One of the benefits of early neutering of male dogs and cats is to greatly reduce “serious” humping. It does not completely eliminate the behavior though, as there are other causes besides a desire to reproduce.
|Nanu and Suszu - also note the "demon eyes"|
The most important thing to remember about humping is that it is often not sexual. There are many other possible causes, and one of the most common is play behavior. Dogs, just like toddlers, who don’t realize the social implications of what they are doing, will put themselves in socially awkward situations without hesitation. It doesn’t have the same social stigma to them, and therefor is just playing. It is common to see dogs as young as 8 weeks trying to hump each other, and this is months before they would undergo puberty, and even have an understanding of what they are doing.
|What's love got to do with it?|
Dogs will also hump each other as a form of establishing dominance. This is not as simple as to say that a dominant dog will always hump a submissive dog though. With some groups, if the dominant dog is very secure in their position, they may allow the submissive dog to hump them as play behavior. Since there is no question about who the dominant dog is, the humping does not have the same connotation. Some dogs will even hump objects as an attempt to establish a pecking order. I’ve known many Yorkies who carefully rule over a pack of stuffed animals this way.
Humping of people and legs can be play, or an attempt at establishing dominance, but often it is attention seeking. Many dogs have learned that while jumping around and making noise can be ignored, few people ignore a dog humping their ankle or shoe. As any behavior that always works to get attention, it becomes more and more often repeated.
Rarely, a spayed or neutered dog will have a medical issue that causes hormones to be secreted, and may lead to a sex drive similar to what an unaltered pet will have. This behavior needs to be checked by your veterinarian, as it can indicate a severe medical condition.
Having your dog or cat hump your house guests will never be really acceptable. Perhaps some of the blushing can be reduced as you explain that Fluffy is really just playing, and not actually amorous.