at 6 Jun 2006: 11:21
I feel compelled to comment on your assumption that dogs benefit, physically, from breeding, and also that you erroneously believe that a female dog having a litter before being spayed is a good idea and will prevent reproductive-related cancers.
It's quite the opposite actually. Female dogs should be spayed at 5-6 months of age and should NOT be allowed to have a litter, nor go through a heat cycle because estrogen is stored in the body and thus it increases the chances for mammary and uterine cancers later in life. There is no benefit to having a female dog have a litter before spaying, and to encourage the mentality that there is some sort of benefit just foolish.
Also, there's no evidence which suggests that dogs receive some kind of physical boost (immune boost included) by obtaining an orgasm. You're referencing a human biological trait. Humans are hard-wired to have sex at any given time because humans have sex for recreational purposes. Canids don't have recreational sex and are not hard-wired to breed that often on a continuous basis (you figure a female dog comes into heat only once every 6 months or so, and is only receptive for a few days).
Maintaining a dog by jerking him, or her, off isn't benefiting the dog in any way. Rather, it displays to the dog that you are a willing, and receptive partner with whom he or she can reproduce with, so naturally the behavior continues.
I have to say, I thought about this for a good deal. I'm curious how many self-proclaimed zoos are having sex with neutered and spayed animals...animals which are NOT acting based on the instinct [need] to reproduce. It would seem to me that, with the desire to reproduce gone, an animal that continued to have sex with his or her partner, is truly doing so because they enjoy the activity, whereas the unnetuered or unspayed animal is acting solely because of instinct and the drive to reproduce.
But that's getting off-topic for my particular response...which is simply to inform that allowing a female to have a litter before spaying is NOT beneficial to her health, nor does it prevent reproductive cancers later in life. Likewise, allowing a male dog to ejaculate a few times a week doesn't benefit his health nor prevent cancers either. The only thing that prevents reproductive cancers and diseases is the removal of the hormones which trigger them...testosterone and estrogen, respectively, and the only way to remove those hormones is through neutering and spaying -before- they're prominent in the body (sexual maturity).