Were it me, I wouldn't explain anything to her unless she asked nicely. I just don't tolerate demands well.
As for how to explain it, well, it's just that you find yourself loving multiple people as a natural extension of who you are. Those other people whose lives you're supposedly ruining know all about and make their own choices, so your mother can rest assured they're choosing their doom.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.