I (the female half) brought up poly to the BF about a year or maybe two years ago. He asked me a funny question: "If you came home and saw me with another girl what would you do?" I answered with "I would join in!". Two things lead me to this answer. 1) I trust the BF 100% and if he found another girl that means he loves her deeply and who am I to stop true love? 2) I'm not a jealous person. I have never felt jealous when it comes to the BF loving more people than just me.
However, the BF didn't like my answer as much as I thought he would haha. He thought I didn't love him with all my heart because I wasn't jealous. But after time and explanation he came to share my idea ^_^ (I've been poly since I was 17). The BF and I have a fantastic relationship. I feel as though it only makes sense to keep building that love. Having another person join our relationship just seems right. Now that the BF's and my relationship are sturdy and happy, it should be able to easily support another heart ^-^.