As a parent of two tiny ones, I see this stuff so much that I don't even think about it anymore. The only reason I let my wife take me to parenting/child groups is because of exactly what you just said. I don't think I have gone more than a week without seeing butt cleavage or bare nipples in oversized bras.
I am finding that a lot of women are quite aware, but they feel safe in such an environment, so they truly don't care. So, I don't really care that much, either. If I saw this exact thing at the mall, then I would go nuts, but in these playgroups, I actually don't even flinch a muscle. It's interesting, too, how many women start out nursing under their covers for a few weeks, then after a while, none of the women use covers anymore, and they don't even try to hide anything when they are latching-on...they literally "whip them out" for several seconds before the baby is even ready, knowing that other dads are looking, and they don't care one bit.
Hmmm, I actually feel like a creep writing all this out, since I experience this every week...heh. But so be it.
