The question is how Pink fits into this, since that description didn't mention _another_ partner at all.
She doesn't, because "this" is "the potential marriage between stevie and Blue" and that, at its core, is an issue that is only between stevie and Blue.
All this stuff about how it "affects" her and "where it leaves her" etc. is putting the cart before the horse. Fact is, Blue and stevie getting married only affects Pink if Pink and/or stevie let it. e.g. if stevie changes the way she behaves towards Pink, or if Pink changes the way she behaves towards stevie. And in that case, the effect is secondary, in response to the change in behaviour, not primary, in response to the marriage itself.
At the end of the day, what matters is needs and feelings. Pink's needs being met are not affected by whether or not her girlfriend is married. They are affected by whether or not stevie starts treating Pink differently, not by whether or not she starts treating Blue differently.
People have a general need for equality, but that doesn't mean being treated the same. It means not being treated as inferior. There's nothing inherent about two people being married that causes a third person to be treated as inferior. Auto and I, Zoffee and Cue, we don't treat each other as inferior simply because we happen to be married to someone else. We respect each other's differences and needs and feelings, as being unique and individual. We allow each relationship to grow and evolve on its own merit.
If it was ever a possibility that Pink was going to marry stevie, then we might have something to talk about, because marrying Blue would kill that possibility. But from what we've learned, that's not on the table at any point in the future regardless, so it's hypothetical blockade is irrelevant.
Sure, marriage is kind of a big deal. But if Pink and stevie's relationship is so tenuous that this marriage could make them treat each other differently enough that their relationship falls apart, then there were deeper issues to begin with.