I just feel that we need to break the mindset that Trump has unqualified support from white guys. I think it would be beneficial for a lot of the Trump voters who were very uncomfortable voting for Trump but did so anyway from latent misogyny and racism (among other reasons) to see that "look, you can be a white guy and totally oppose Trump. it doesn't make you less manly or anything. Lots of white guys hate Trump."
It also denies the media narrative that this is a "white guy v. everyone else" issue. Lots of white guys voted for Hillary. Lots of white guys have been opposing Trump. The media should not get a free ride on framing this as "white guys support Trump." It is a particular subset of white guys (and smaller subsets of white women and Latinos).
OTOH, this is classic conservstive MRA tactic of setting things up as an opposite. "Oh, you wanna say Black Lives Matter? I think All Lives Matter!" This could easily be perceived as trying to take agency and leadership away from women and the Women's March.
I think the other marches help to push back on this perception. We now have climate deniers and scientists planning to march, and giving credit to the Women's March for inspiring them. Another follow on March explicitly crediting the leadership of the Women's March and with a clear statement of goals -- while not eliminating the potential for misinterpretation -- could also concievably reenforce that, yes, there are plenty of white men who are quite happy to be led by and inspired by women.
But the arguments I saw play out between folks on whether men should or should not participate in the Women's March have somewhat disheartened me on the subject. I am not sure I am up for the inevitable dissent nd argument with people with similar or identical overall policy goals. This is why I always say "I am no one's ally, I am a fellow traveller."