I'm glad you're willing to explore this with your guy and that you're here.
I am married and two of my other partners are as well. My husband also has a partner who is married. For me, it works because of compersion.
The I define compersion as taking joy from my partner's joy. For example, when I know that my husband is with his partner and they are having a good time together, I feel good. It makes me happy to know that he his happy with someone else. It makes me happy to know he loves other people.
Another part of it, for me, is that I know I cannot be all things for one partner. The way we live in the Western world these days, moving around and moving away from family and established friendships, we often place the burden to be our emotional support on one person. Because I know that I can't meet all of my husband's needs, I am happy he has found someone who can fill things I can't. An obvious example of this is that we are both bisexual. Since he's not a woman, he can't fill the spot in me that years for a woman.
It can take some time to wrap your head around polyamory if it's new to you. Please be gentle with yourself and try not to rush. That never works.