Thanks for all the replies! Still thinking about it, and talked about it with some poly friends..
I've used "Why not?" in the past, but I don't really like it. (I've also used "because I can", which I like even less now). I don't know, it seems... too easy?
I know there are so many different kinds of poly, which is why some of the answers on this thread would not work for me.
The point I would like to get across, when asked, is: that this is an adventure, a journey that has taught me more about myself and relationships and love than 20 years of monogamy, a challenge, an opportunity for growth, a chance to intimately get to know amazing people. But if I say that I sound like a thrill seeker or a someone who thrives on drama, while this is not the type of challenge I mean at all...
early forties, straight.