I actually agree with Zirin (and am jumping up and down and saying "look, I'm a conservative!" when I say this) that differences in gender can be minute. There are "fluctuations," as he says, given the sheer number of intersex occurences. But acknowledging that these fluctuations exist means that you simultaneously acknowledge differences exist. Pretending like those differences amount to nothing more than aesthetic peculiarities (or as Zirin says, don't amount to real differences in things like sports) really undermines his point about accepting these differences and doing a better job of working with them.
Perhaps gender testing is indeed necessary. It's a messy idea and one sure to garner lots of controversy and hurt feelings. But ignoring it alltogether only serves to make you more ignorant of the very thing you're fighting for: better sensitivity, and better management, of gender issues.