Oooh, this is a difficult one.
I have some issues with breast cancer charities myself - but I acknowledge that my reasons are personal and a bit irrational. Breast cancer research gets a great deal of money when compared with other kinds of cancer. Of course, the money has been put to great use, and many fantastic new therapies are available and are saving lives every day, which is fantastic. However, as someone whose mother has lung cancer, I do have moments of anger when I consider the huge resources devoted to breast cancer in comparison to other cancers, particularly when breast cancer survival rates are so much improved. (My mother's oncologist reckons that breast cancer receives something more than 50 per cent of all cancer research funds raised in the UK, while lung cancer - the biggest cancer killer - gets less than 5 per cent.)
However, I suppose the answer is to expend your energies on raising funds for whatever your personal concern is. I'd say that morally, you're entitled to refuse to take part - but you might want to spend an equivalent amount of time fundraising for charities that investigate the causes, if that's what exercises you. Also, be prepared for lots of people to think you're a bit of a nasty piece of work, and to spend a lot of time justifying your position!