I've given the smart ass response "because that's how we roll" to people before. But usually I tailor my response to the person I'm explaining it to. I'll get into as much detail or explanation as people want. I found most people are just confused and unaware that polyamory can even work. I've recently been more "out" to coworkers and friends and they've got lots of questions.
"you're still with your husband?"
"and you have a boyfriend?"
"and they both know about each other?"
"how does that work?"