I totally got what you meant, the problem is with what it comes across as. I'm sure you meant it innocently enough but it can come across as somewhat insulting to others.
I'm married and have a boyfriend. I could say that my boyfriend is an additional relationship. Well all relationships are 'additional' to the ones we already have. However, saying he's my additional partner makes him sound like that freebie sample size package you get when you buy your favorite scented cleaner. He's an extra, just added on. It sort of demeans him.
While you'll find plenty of people here that have spouses and non spouses, couples that have a primary/anchored/established relationship, most learn at some point that it's a matter of changing your thinking.
My boyfriend is not an ADDITION, to anything. He's a person in his own right. Our relationship is our relationship, not an add on, a bonus package. Thinking of my relationship with him in terms that come back to my marriage is a tad rude. I don't think of my husband as an addition to my boyfriend!
It's the idea that your established relationship is THE relationship and everything else must fall in line to that. No one wants to feel that way. NYC will often chime in because, IMHO, she is one of those people here that is NOT in an established relationship that then decided to be poly. She's what some refer to as 'single poly'. So is often the one people think of as 'an addition' so yeah, when you get the brunt of that kind of thinking it kind of irks you. She's not an addition, no one should be. It should be a relationship, on it's own. Not an extra or discount to an existing product.
I really hope I'm making sense here. No one means to be so 'anti couple' here, but it happens and it's frustrating, but it's also understandable. Many couples come to poly and for a sense of safety, they use the whole couple hood as a safety net. That everything else is AN ADDITION to their couple. I get it, but it's still insulting to others. Honestly, hubby and I have never tried dating as a couple, we don't search out a third or a quad. If it happens fine, but we are individuals and date as such. Metamours might never even meet, there is no besties, there is no need. BF is not dating hubby, he's dating ME. Sure he and hubby have jokes and talk through me, they have now met, and BF is part of my life so gets to hear about the kids and work and all that mundane stuffs, but he's not an ADDITION to our family, our marriage.
PS The comment was for NYC and others that were shaking their head at the ADDITION stuff and hoping to lighten things for those that might be frustrated by it. It's not a bad thing, I dont think anyway, but it's just something people don't think about until someone points it out.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year