I'm not in a poly situation right now, but when I was, the guy's girlfriend and I look(ed) nothing alike. We're very, very different in looks.
I'm petite, dark-haired, brown-eyed, and thin/toned in body.
She's seven inches taller than me, blond-haired, blue eyed, slightly heavy/a bit meaty. Not fat, though. I can tell she doesn't watch her food intake as much as I do nor does she exercise as much, but she's meaty/slightly fleshy, not fat.
In a way, I thought it was bizarre that he even went for me if she was his girlfriend (and presumably his type). But I guess some men like a variety of different types.