This is not about whether men and boys should wear bras. This is about a transgender activist lobbying a small company set up and run by a very young woman which until now has made young girls its target market. I've attached a screenshot, which I saved from Lily Maynard's blog. The activist is explicit that it isn't acceptable now to say something is just for girls. S/he thinks the company should use gender neutral language about their products so that they are more inclusive.
As Lily has said in her article, are they similarly lobbying all the big clothing retailers and asking them not to do this? And if not, why not? Could it be because they thought a young woman and a small, growing company were much easier targets?
Final point - I for one would be much happier to think that a young girl struggling with having an increasingly female-looking body would wear a very plain, unwired, unpadded, unsexy bra instead of binding her breasts to flatten them. I doubt it will happen though.