The other side of the fence
I realize some people are just genetically monogamous. But if that's not you, here's yet another reason to not be afraid of opening up your relationship - it makes you appreciate your SO even more.
It's a cliche that the grass is always greener on the other side of the fence. It's because you can't try that grass, so you end up idealizing it, and nothing in real life can compare to the ideals humans can imagine. It can drive a person to distraction, this constant feeling that you're missing out on the best grass evar. And it's right there, just in reach, but forbidden. Argh!
This sort of thing is toxic, but there turns out to be a really easy way to counter it. Just go and have some of the grass on the other side of the fence. You will realize that while it may be really darn good grass, it is still real-world stuff and not magical fairy food.
This metaphor is starting to outgrow itself, so to bring this back into the realm of the literal - when you develop extra-primary relationships, you reground yourself in reality. I have found that I appreciate my SO even more now. I'm not comparing our relationship to a Hollywood fantasy, I'm comparing it to other real people, and I notice all the things about her that make her ideal for me. No disrespect for my other partners, of course! I like them a *lot*, obviously. But you forget all the little things about your primary that makes life with them exceptional, and being involved with someone who doesn't fit you as well reminds you in a hurry. I'm sure my paramores feel the same way about me vs their SOs. And the fact that we can all help each other remember what makes our primary so magical to us is a beautiful thing.
So, as I've said before, if you're already with the right person, having an open relationship tends to make your relationship stronger. Don't be scared of it.
Also, it really is nice to get that variety. :) No matter how much you love pinot grigio, a bit of sangria now and then to shake things up is vital.