Well, I can't explain poly to a mono more than to say:
It's like with any friend. This one is fun to shoe shop with, this one is fun to watch a movie with, that one is fun to go hiking with, this one is fun to talk about religion with, that one is fun to talk about our kids with, this one is fun to cook with or do a craft, etc etc.
People are different and each friend we have fills a slightly different need. Personally I'd never be so arrogant as to think I could completely fulfill every need my gf has for what she looks for in a friend. So, with lovers, it just takes that and adds the sex/romance bit to it. Variety is the spice of life! New lovers can bring you new sexual tricks that you can then try out with your primary. New lovers can bring you a different perspective on life that can enrich your short journey on this plane. Ideally new lovers will enhance and enrich your relationship with your primary, not threaten and diminish it.
Of course, in my case, I am a cis-gendered* woman, and my gf is a pre-op transwoman. If I date a man, I get a guy who likes his cock and likes using it (she dislikes hers and wishes she had a vagina, so this hampers her enjoyment of sex sometimes). If I date a woman, I get to play with a vagina (tho I have not dated a cis-gendered woman since my gf and I met).
When my gf dates a guy (she has a bf now, since last November and is in love with him), she gets gender affirmation and the thrill of looking like a heterosexual couple when they are out alone together. (Before she started transitioning, she was living as a man and when she had a gf, they had the appearance of a het couple, but with her as "the man," it never felt right. And if she dated a guy, feeling like a gay male couple never felt quite right either.)
*cis-gendered-- born with a body to match the gender in your head
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
There's no lying in polyamory!
I'm a 58 year old woman with 2 partners:
miss pixi, my live-in gf, 36 (together since Jan '09)
Ginger, bf, 61, married, lives nearby (together since Jan '12)