There are a bunch of us who don't have anybody "on the side." Were we able to marry freely, we would, and all of our partners would have the same legal status. I have a wife and had a girlfriend because I'm married to the one and got involved with other later; were it possible to add to our marriage, we would have been planning for that after a suitable engagement.
Nothing I've said on the matter has been predicated on any specific type of relationship. If we treat our relationships as matters-of-fact and nothing spectacular in any fashion, then others are more likely to do the same. Whether you consider all partners primary or have primary, secondary, and tertiary partners matters not.
And remember that one can be quietly out of the closet. As I said, some people heard about my wife and others heard about my girlfriend and others heard about both--all as the vagaries of conversation brought them up. I am convinced that exactly that sort of approach is the most effective at opening most mono people up to the notion that poly is OK and nothing about which they should be concerned.
If folks don't find us threatening, nor the way we act threatening, then they're much less likely to find the notion of polyamory threatening. Tossing it in their faces makes it threatening.