I think it's partly because we BF for such a short period of time in our lives (if at all). For the rest of the time we do tend to view them that way. We often wear nice underwear, get bombarded with imagery (ads/ films/ etc/) and for a lot of people they are a source of (s*xual) pleasure.
So I can understand why we see them that way, and why some of us find it hard to switch off that thinking when we have kids.
I struggle with it. I wish there was an easy 'now they're for kids' 'now they're for me again' switch in our brains!