Your experience in union negotiations seems to be focused more on manufacturing. Mine more on union professional, para professionals and service workers. So that may be part of the difference we've seen.
In any case, while union agreements have gone a long way toward shrinking the gap, it still hasn't brought us equal pay....or maybe it depends on how one defines equal pay. Studies show the gender wage gap in union positions is about 10% while non-union positions about 20% (2013). Of course the gap is larger for "minority" women. I suspect much of this has to do with which jobs are considered 'more important/more skilled" And clearly, as it relates to seniority, it the issue that women are the primary caregivers in the US (and most ofter places.) So men work more, longer hours and tend to have more 'seniority.'
I'm not saying that unions aren't great...What I'm saying is that there are societal issues that this doesn't solve. And working on issues in one area, often leads to complications in others. (We haven't even touched on the question of whether an employee who just does better work should be better compensated...a completely different component of the 'equal work for equal pay' discussion.)
Let's be clear: In a union contract, there are NO differences between sexes. All are paid the same. Now, the different jobs may have a difference in pay, but again, if you are doing the same work, your pay is the same per contract. Say when you start, you start at the minimum, and in two years you hit the max. That's the same for men or women. I don't have a problem with progressive pay scales. I'm not a socialist who thinks everyone in America should make the same wages. I started at the bottom, got raises, took training, and moved up.
But if you are such a person, and just thinks everyone in America should have the same pay, well, okay. We just disagree.
There's nothing wrong with that, in my opinion. The problem is in non-union workplaces, where the employer doesn't want you telling each other what you make, because they are paying everyone different, and paying men more than women for the same work. In a union workplace, all the wages are out in the open, and I think that's a good thing. Everyone knows, and you're pay is per a formula in the contract, and everyone is treated the same in pay, male or female, white or black, gay or straight.