So, you're not the sort of person who can easily enjoy casual affairs? Having a lover or two on the side can be a wonderful distraction when the secondary relationship in your life isn't filling up all the spaces you have that you want to share. If you're really committed to Mac and don't see the opportunity to be co-primary to him along with Lisa any time soon, having someone else you can share a piece of yourself with may be a strategy to not go insane.
I totally empathize with your minivan thing -- I've frequently felt that sort of difficult envy too with my girlfriend and her husband. It's gotten better as they' ve felt more comfortable with me being a helpmate to them after the birth of their baby. And, as it happens, I fell into a monogamous relationship on the side that might turn into something primary but that doesn't prevent me from being with my gf.
Good luck!!! Being a secondary is tough and blogging has definitely helped me. There are good resources on the subject of secondary relationships at www.morethantwo.com
. I hope that you and Lia can repair your friendship, it seems like that would help a whole lot.