Why is the female of the species called what we consider an offensive term? Lets look at how this came about.
As it happens, as far as research tells us, the term bitch was used first to name a female dog (much like a mare is a female horse). The slang, offensive term often used against women, actually came much later - its use was to degrade and suggest dog like qualities, like a female dog on heat - bad tempered, a fixation on sexual urges.
A dog is a male dog (intact or neutered), a dog that has been used for breeding can still be referred to as a dog, or a stud. If a female dog has been spayed or not, she is still called a bitch.