Why are they Marching?
To highlight that we have a thug with an adolescent temperment for a president.
To bring attention to the fact that under his agenda, there is a very real threat that womens' reproductive rights will revert to the stone age.
To protest the fact that soon 20 million or so will lose their health insurance. Another 20 million or so will be priced out of the market.
To protest the threat of dismantling NATO.
To protest the idea that we need to spend more money to expand our military. Already bigger than the next six national militaries combined.
As for reasons, I think that is a start and well worth marching for.