I think there are a few differences between this and harrassment.
First, participation is voluntary. No one has to do it.
Second, if you don't like reading the updates - you have the option to defriend anyone who behaves in such a vulgar fashion. Unfortunately, for me this would be half of my Facebook friends who are female - over 25 people, I think.
Finally, it's fairly subtle. Unless you ask about it, you won't know what those colors mean. And just saying a color isn't really an issue - it's only once you know the context that it becomes embaressing.
However, I didn't see the point, so I didn't participate. This sort of "gag" might work for a condition that no one has heard about, where people ask about the colors and learn about the existance of a relatively unknown health condition that is still important. For example, someone suggested having guys post boxer colors to raise knowledge about prostrate cancer, which is nearly as common as breast cancer (if I recall correctly), and more fatal (again, if I recall correctly), but there are lots of people who don't realize how serious it is. (But then, guys talking about their underwear is "threatening" or at least "gross". When women do it, it is "hot" and okay, they are doing "what guys want". Our culture is just really . . . confused)
The key thing, IMO, is that everyone knows breast cancer exists and is a big problem. All this did was get people saying, "Breast cancer exists", for the most part, plus a lot of giggling. I only saw one person use the gag as an opportunity to raise conciousness - and she did so by posting links with info on breast cancer & contraceptives and abortion, not by posting her bra color. Basically it was her way of saying, "THIS is how you increase knowledge about breast cancer - not talking about bra colors."
I think this was a good example of "group think". It sounds like it's doing something good whether it is or isn't, and everybody sort of turned off their brains and went along with it.
This concludes my over-thinking on this issue. It was a fascinating phenomenon, though - I don't think I've seen so many people participate in a Facebook status meme in a single day. Especially from so many different groups of friends - only one sub-group didn't join in, and they were still clearly aware that it was happening. Strange . . .