So recently we've had a little rash of Belfort Bax links and a reminder of the early (19th century) anti-feminist writings by that socialist. And I mentioned that of course, the labor movement itself (ie socialism) was the first men's rights movement (preceding any sort of women's rights movement). The labor movement was in large part about trying to improve working men's rights in the terrible conditions of exploitation brought about by the rise of capitalism following the industrial revolution.

That's just a preamble.... I was talking to a libertarian recently who opposed any action on the far greater inequality between rich and poor, while of course being an MRA and advocating for action on the inequality between men and women.

This is inconsistent to me. If your attitude towards economic inequality is that it doesn't matter than why bother about the far smaller differences between men and women? Libertarians tend to use the self-contradictory concept of so-called "equality of opportunity". But equality of opportunity can only be possible with a prior equality of outcome. If you think everyone should have the same opportunities from adulthood that means you require equality of outcome from their childhood. If you only require equality of opportunity from birth then you require an outcome of equality at birth itself.

The libertarian argued that being born male vs female was an inequality of opportunity, but then how is being born into wealth vs poverty not far more so?

This is what I mean when I say that equality is a left wing principle (and therefore feminists are right wing). Libertarian MRAs want their cake and eat it. They want to call for equality on sex but not on other much more significant inequalities and there's no principled reason for this different treatment.

Born male? Suck it up. Just like you'd have to if you were born poor. It's a bad start that's all. If you agree then you should agree for both. I disagree for both.