There is a great side-benefit to having multiple romantic partners: you might make new friends with your partners' other partners (also called "metamours"). These are the people that someone that has a very good taste (obviously, because they choose to be with you) likes and trusts. Chances are, you'll have something in common... besides similar romantic taste. :)
Metamours are more than random people you meet - they care about the well-being of someone that you also care about, most likely they wish for your parter to be happy, just like you do. They are almost like distant cousins that have family's best interests in mind. They enrich your partner's life in unique ways, just like your other partners enrich yours. That's partially why we are polyamorous: we want to connect with people that have different life experiences and come from the different backgrounds. We learn something new from everyone we meet and every partner makes us feel special in a unique way.
I am also extremely curious what kind of people my partners are into - just like I am curious about their friends, hobbies, jobs, favorite foods, likes and dislikes, and the other important aspects of their lives.
The first time I met my husband's girlfriend was very interesting. I was slightly nervous, but not nearly as nervous as she was. We got together for a BBQ at our house. When she walked in, I thought she might faint - meeting your partner's wife for the first time is a pretty intense experience! I immediately poured her a glass of wine to help her relax a bit and at the first chance I got told her how grateful I was that she was part of my husband's life and how happy I was that they had that special connection. By the end of the night we felt way more comfortable with each other, and I satisfied my curiosity about a new person in my husband's life.
Metamours also help my partners learn things about themselves that can help improve my relationships with them. They learn to overcome barriers they might have and bring that knowledge into our interactions. There's a great article on this topic on morethantwo.com.
Meeting metamours also helped me work through insecurities and fears I faced. When another person in your partner's life is someone without a face or a name, it's easy to fall into the loop of "what ifs" and insecurities, but as soon as you meet them and get to know them, it's much easier to process any uncomfortable feelings. Meeting them helps me understand why my partners like them, reminds me that we are all different people and bring different experiences into each others' lives.
If you haven't met your partners' other partners yet - I highly recommend it!