I have another one. For those who have primary partners: DON'T think the dynamic of your primary relationship will not change. No matter how you attempt to control (or wish to control) the feelings, behaviors, or attitudes of your partner, nor how you may attempt to limit their activities or time spent with a secondary or non-primary relationship, YOUR relationship will never be the same. It cannot be stagnant anyway, but the fact that your partner is intimate with another will change the dynamic you previously had. This is not a bad thing. It should be expected, not avoided.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein