Polyamory is an relationship between three or more people, in which its members approve and actively participate in the sentimental bond with all parties, in a shared manner.

That's what you found on the internet. You never actually knew this existed, and yet you participate in a love relationship like this. You have two boyfriends, Yuichiro and Muichiro. At the same time, and yes, they know it, they didn't care even a little, they just love you and know the fact that you love them both.