I've stated before that while I'm not anxious for the ERA to pass, I wouldn't mind if it did. To me, it seems like most everything feminists wanted the ERA to do, they've managed to get through other means.
But after reading this post at the Volokh Conspiracy, I've come to realize there could be some unintended consequences, and not just unisex bathrooms.
Specifically, Ilya Somin discusses some consequences that feminists probably wouldn't like, such as women in combat and inclusion in the draft, and restrictions on Title IX, which has been most helpful to women in college athletics. It might also abolish civil unions, which discriminate against opposite sex couples.
Eugene Volokh also has an interesting companion piece on the effects of the ERA. He also describes the system in place currently for determining the constitutionality of sex-based statutes.
The formal test is that sex classifications are constitutional only if the government shows an "exceedingly persuasive" justification, which consists of showing "at least that the [challenged] classification serves 'important governmental objectives and that the discriminatory means employed' are 'substantially related to the achievement of those objectives,'" and that the justification "not rely on overbroad generalizations about the different talents, capacities, or preferences of males and females." In practice, the test is very hard to meet, so it's hard to point to many sex classifications that are now constitutional but that the ERA would clearly or even likely ban.
As I pointed out way back in this post, sex discrimination gets intermediate scrutiny by the courts,a standard that is below that for race and other classifications (strict scrutiny), but above rational basis.
Volokh says that ratification of the ERA could create some unpleasant consequences for programs such as sex-based affirmative action and women's sports programs.
Both articles are fascinating reading.