Absolutely you can be polyamorous and in a monogamous relationship.

I think some people conflate "being able to love multiple people" with "REQUIRING to be in a multi-partner relationship". If people feel the latter, it's because they are lacking something in their life. The way someone who is single might feel they "REQUIRE to be in a relationship". Whereas a healthy person can be like "I'm ok and enough as I am, but if I love someone I will want to be in a relationship with them" and in a poly setting "I'm enough in my current relationship but if I was able to fall in love with someone else I would be happy to include them in my life".

I see it kind of like bisexuality. I'm bisexual. That doesn't mean I have to date multiple genders at once. It just means I am capable of loving more than one gender. But I am also capable of commitment and monogamy based on my life and context.