No, Really, Not Wanting To Lose What You Have Is Wonderful.

“So you play a lot at the dungeon,” I asked.  “You’re clearly very sensual and body-oriented.  Can I ask why are you sexually monogamous?”
“Couple of reasons, some bad,” she replied.  “The bad one is that I’ve got it very good with my partner and don’t want to screw things up.”
“Stop right there,” I said.  “That’s a good reason.  An awesome reason.  Don’t you dare think that’s a bad reason for being monogamous.”
One of the things that pisses me off about poly folk is their insistence that poly is some higher level of relationship, and if you can’t hack poly, then you’re a lesser form of being.  This is usually expounded the loudest by people who are trying very hard to get into your pants.
But monogamy isn’t better or worse than poly; it’s simply a different dynamic, and trying to judge which method is superior is like having heated debates on whether the fork or spoon is more awesome.  It’s all about what you want.
And look, even within poly relationships, there’s a limit.  I’m always sexually curious, and I have some new deep and caring friendships who mean the world to me… But right now, I have a wife and two wonderful long-term girlfriends and a serious dating partner.  That’s really about all I can handle.  It’s difficult keeping that many relationships spinning at times, and adding one more serious, full-time relationship would probably throw everything out of whack.
So in a sense, I’m in the same place as my friend: reaching for another full-time girlfriend would probably spoil this very good thing I have going here, and I don’t want to risk that.  And I can’t see why that’s bad.
Not wanting to be polyamorous because you’re afraid of losing the solid, loving relationship you have now?  Valid.  Wonderful.  Eminently defensible.  Nobody has to be poly, and anyone who tells you otherwise is selling you something.  Probably something that’s not worth too much in the first place.