Good point: pursuing romance without discussion first is dishonest. But every couple has different expectations. Asking if she's interested in a romantic relationship is not the same as an emotional affair. What if she says no? No use asking your boyfriend for an open relationship (if that is what you're after) without finding out if this woman you're in love with likes you that way too.
Another good point: threesome-seeking lover and polyamory are a bit different. Not saying one can't lead to the other, but they do have different bounds. What I said to begin with about pursuing a relationship first is what I stand behind. If you love her, it would be unwise merely to serve her up as a side dish to your main squeeze. Secondary partners have feelings too.