It's a tricky situation. For the mono party it's easy to see things in terms of quantity...you get more partners, more sex, they get less time, less attention. What they're missing out on though is the quality. Because you are able to express yourself freely and are not confined, or as confined the quality of the time you spend with your primary is (hopefully) better. Yes, your heart is more open to other people, but it is also more open to your primary partner and love is not a finite resource, even if time is.
The other side of that is polyamory is a part of you and so if you are living a poly lifestyle and your partner still chooses you then you both know that he is choosing the real, honest version of you.
When my previous partner found out he tried to be open to it at first. I think he was fine with the idea in theory and before we were together he usually had several partners at once, though he never identified as poly. However, he had a difficult time dealing with the physical reality of what that meant and as time wore on he instituted more and more rules about who I could and could not see and what I could and could not do with them. In hindsight I probably should have been firmer about what I was okay with.