That's not necessarily true. As I point out elsewhere, there's much more going on than just ones top marginal rate. I pointed out the discontinuity when one hits AMT.

But also, more topical to this aspect of the argument, is the other expenses involved in employment. It may be that I'm still making 60% of my labor after tax. But in so doing, I also have to forgo the work that I'd been doing myself, and pay someone else to do it.

Most relevant is childcare, since it's something that one can't readily avoid if both spouses are to be employed, and is extremely costly. It's quite possible that the loss to taxes plus the expense of childcare together exceed the potential income of the second spouse. Indeed, I'd guess that is a leading reason for single-income families.