Like many, it is just a natural reaction. All of us do it, whether we are straight, Bi, or flat out crooked. I admit to doing so as well, but only if I find the person attracting or if I just have the urge to look. I see nothing wrong in doing so since it is only natural. Plus, if we don't do so, then we might as well lock ourselves up in a room with nothing but four walls, because what else can we do if we really don't "pay attention."
As for with a significant other, currently I have no one. However, whenever I do I really wouldn't mind, as long as they don't do anything they aren't supposed to... Though, I don't blame them for looking because like I said, I would do it too.