I think my problem is that it feels like you're focused on the sexual part of girlfriend as a relationship label. I don't see how it's any different than wife - I think most people assume men fuck their wives, and vice versa (regardless if that's actually the case), but I don't know that you'd say that to a four-year-old..."This is my wife. That means we have sex."
I don't see how being honest with kids endangers their well-being, but I do see how being dishonest or unforthcoming could hurt my kid. I don't choose to perpetuate secrets or gloss over reality, and I don't want him to grow up to be ashamed of anyone or ones that he chooses to love. I strongly believe that teaching love, teaching, tolerance, teaching truth, teaching compassion are our most sacred responsibilities as adults who influence children. Trying to hide, or to gloss over someone's loving relationships sends a message that it's wrong, bad, shameful, or something not to be discussed.
I'm happy to agree to disagree, and I think you and I probably live very different kinds of lives that color our perceptions.