I'm with Fallgal, I could have sworn I've seen men on those walks before. I don't see why they couldn't participate or why they would be discouraged. Men can get breast cancer too... It could be your particular section of Pink? Maybe they have one particular race just for women and another mixed? Or maybe your MIL was incorrect?
For some reason while reading your post I couldn't help but recall when I was talking to a male friend about feminism and the idea of male feminists astounded him. "How can there be
male feminists?" One of those palm to forehead moments
I personally don't think you're making 'too big' of fuss over it, I think it's a worthy topic for good discussion. On that note, it doesn't sound like your DH and MIL are sexist, maybe they haven't been personally affected by breast cancer or knew anyone close to them who has suffered from it. I don't think a son who has lost his mother to cancer would think it's only a women's issue. If their only thoughts and exposure to breast cancer have been Pink charities, mostly run by women wearing pink, that could be a reason why.
That said, if your section of Pink is restricting their walks to just women, maybe contact them and say that perhaps their exclusivity (especially since they're a very well-known charity) is discouraging men from joining or showing their support. For me doesn't make sense for a charity that is so well-known in the mainstream to be exclusive :/