"Ok. I attend group classes, and it is extremely unfair, inconsiderate and just out-right rude for people not to rotate partners when the instructor tells the class to do so!"
Tough luck. You don't know everyone's personal situation. It may be that staying together is not the best for their dancing, but ultimately the personal relationship is more important than the dancing, even in a couple that is only dancing together. Dance issues can be fixed later when they get to the point of private lessons, but interpersonal problems could well be permanent. If staying together is what makes them happy, then that's what they should do - regardless if it's the best in the long run or not.
"Sorry if I come off a bit angry, but, it's really annoying when you have to dance by yourself several times in the class simply because some people are making the conscious choice not to rotate."
Maybe you should make an effort to help recruit for the class.