After sexual intercourse, why does the penis of a male dog get stuck up the vagina of a bitch? Why doesn't it happen in humans?

From the recent (Feb 3, 2004) NOVA on PBS about dogs, I remember this subject being mentioned. I seem to remember the reason for the prolonged copulation after insemination is to cause a delay in the availabilty of the female to other males, improving the copulating male's sperm's chance of successful fertilization. Once the ova have been successfully fertilized, any additional insemination will be superfluous. This prolonged copulation after insemination is caused by the physiological structure of the dog's penis, and corresponding "locking muscles" in the female's <a href="http://www.livestrong.com/article/12495-the-vagina/" title="vagina" style="" ,="" class="stronglinks" target="_blank"></a>vagina. Slangily called a "dog knot", the male structure is a bulbous globe of erectile tissue near the end of the dog's penis, and it engorges and, along with the female's locking muscles, precludes (or makes difficult) the withdrawal of the penis immediately after ejaculation. These ...  more