Yep, polyships affect each other. Like they say, love may be infinite but time sure ain't! Not to mention that when I've had a good time with one partner I find that I'm happier and more into things with the other, whereas if I'm feeling confused or down about things with one I'm mopey with the other.
Also, I'm not a fan of OPP (what if you suddenly sprout a penis, would she need to break up with you?) but I can see being lenient with them. As I recently described on my blog, my lovers have had a checkered past with poly. Mistakes are natural, it's all in whether you're willing/able to learn from them...