Now, obviously romantic love and maternal/paternal love are different beast.
BUT most people understand that if you say you love one child with "all of your heart", it doesn't mean you have no love left for the other. You still love all of them with all of your heart.
If you felt that you could only have one child, but your partner had that child with you, and children with other people, it might be harder and more complicated. But surely you wouldn't doubt that he loves you shared kid just because he has others. You could doubt it based on his words or actions, but not just because he has more kids.
Similarly, my father told me when I was younger that a parent will always love their child more than their child loves them, because of the care creating strong bonds. Now, I don't think it's absolutely true since there are completely abusive parents who are still loved by their children, but I understand the concept of it. Even if the love is huge both ways, it's a different involvement and interest in your child's life than they have in yours. They love you, but their life doesn't revolve around you. For a parent, their life might very well revolve around their kid(s).
And while a romantic relationship is between equals and parenthood is not, people are not usually bothered by the asymmetry. People don't generally think "I go to my kid's end of year play but he won't show up for my company Christmas Party" and they don't feel less loved as a result.
What I mean is that I don't think a poly person loves a mono person any less than they're loved due to being poly (first point) but that even if they did love in a different way, it's not necessarily a problem if the relationship works and you get what you want out of it (second point). A successful relationship doesn't require perfect symmetry; it requires people being happy and comfortable in it, and if they aren't, working towards being so.