I was mildly supportive of Men's Rights campaigns a while back, not strongly, but I was somewhat concerned that certain movements were advancing the roles of women (and other groups) at the expense of straight, white males like myself. I would never say stupid ill-informed rubbish such as "the most discriminated group are white males" or something like that, and fully supported equality, but was probably consciously 'worried' that I would 'lose out'.
I think that views such as my own, were/are largely based on rough ideas of feminism, rather than actually reading and listening to what the movement tends to be about.
I (in a sort of nice, but naive way), thought that sexism/racism/homophobia was something confined to 60+ year olds, it wasn't really an issue in the modern world (I'm from Britain if that makes a difference). I thought that if we all just seeked equality, let employers select people purely on merit, give everybody equal rights under the law etc... that would be enough.
But actually, I have encountered people of my age (20), that are actually fairly openly sexist/racist/homophobic. Some that are currently in positions of power, where they can employ others, and may go on to more. I have no doubt that these people would consciously discriminate against women and homosexuals. I also believe that there are plenty more who would subconsciously discriminate in similar positions.
However, more than that, are the circumstances of sexism which is simply inherent in systems and society - it is no single persons fault, it is just the way things are - and legislation to make everybody equal in the eyes of the law does very little to impact upon it. I don't believe that women are any less naturally disposed to be CEO's, or Physicists - yet they clearly are pushed by society, not by direct pressure, but by constant ideas about what a man/woman should be throughout their lives.
This is why feminism solves many of the supposed 'male equality' issues. Men should not be seen as the macho workers, and women as the carers - men and women should be equal in how they work and how they raise children, that is a feminist issue.
In general, I probably need to nail down my opinions a bit more and read more material, but I certainly think that 'Mens rights' just miss what feminism is actually about, or just completely fail to see that men clearly have somewhat of a benefit in society from being a man.