Hulababy the research into this backs up your DH's position (to some extent, clearly there are additional factors).
When women don't ask for pay rises, it's kind of a snowball effect. It means at their next job, they declare any previous salary at a lower level than their male peers. It influences pension payments (often, a percentage of salary), can affect bonus payouts (again, if a percentage of salary), and so on.
Non-directly, it can also boost people's self esteem and confidence at work - there's nothing like feeling undervalued to knock both of them, which again influences how women behave at work, and so on.
Women need to start asking for pay raises to the same level a man does. I once worked alongside another manager who commented (privately, we were also friends before and after we worked at the same company, although we're at different ends of the country to each other now and see each other only rarely) that it's not that women don't deserve the pay raises, it's that they don't allow their own managers to fight on their behalf. If you've got a sqeaky wheel male employee that the company wants to keep and is asking for a justified pay raise, he's going to get his slice of the wage pot for that financial year, isn't he? There's no hope of his silently seething female peer getting one then.
Also wanted to add that no salary negotiation should come as a surprise, to either party. I usually had an idea of what kind of salary increase I could authorise/justify (to myself, the directors) when I spoke with staff at their reviews... this stuff is formed months before the actual verbal negotiation is done across the table.
That's why it's so important to constantly stress your worth. Men do it - but women try and tear each other down sometimes, esp. in Britain, where the "selling yourself" idea is so much more reserved/frowned upon than, say, in the US.
It probably should be noted that I have worked in Sales for a large part of my career, or rather, within a sales environment, so find other sectors' employees far too laid back when it comes to slicing their share of the pie 