The major problem, I think, with this entire situation (the pay gap) is that it has SO MANY FACTORS involved in it, and each one of those factors heavily influence what the outcome is.
1. Child-bearing age women are more likely than men to take maternity leave time off, thus hurting a business
2. Non child-bearing age women are (until I see evidence otherwise) the same likelihood as men to take time off (more/less?)
3. Women unable to bear children are (same as #2) the same likelihood to take time off as men
Those are poorly written examples, but I'm in a hurry. I think you see my point. Basically, its different for each and every single woman on the planet. Some women pop out babies every few years, some do it because they love children, some do it for tax benefits, some do it on accident. Some never have kids. The fact is though, as a business owner, the best you can do is make a judgement based on the information you HAVE. Overall, if you factor ALL women into one pile, and ALL men into another, women are more likely to take more time off. To simplify things for yourself, it isn't unreasonable to want to pay women less for that reason alone. No, it isn't fair to specific, individual cases, but more often than not (aka statistically) it IS fair, so a business owner is likely to do it.
Plus, we have a situation where a business owner looks at this situation:
1. If I fight for equality, I pay more money overall
2. If I allow there to be a pay gap, I pay less money overall
I find it hard to imagine the business owner that would gladly pay more overall for an "idealistic" situation. You know what I mean? Don't get me wrong, I'm not saying its fair/right/wrong/silly/bananas, I'm just saying when it comes to business, if a business owner can make the bad guy the "pay gap" instead of him/herself, then they're going to continue paying one group less. I guarantee you the second that bosses figure out how to lower men AND women's pay overall, but keep the same quality of workers, they'll do it. Hell, they already do. Its always about the bottom dollar, and how low they can get it.
**EDIT** I would like to politely request that, in the future, if my ladyfriend ever browses these forums, you all back me up when I deny my statements here, and pretend fully that I supported equality with a vehemence, and we can all pretend I even yelled at somebody who disagreed :P I'm kidding of course, possibly a poor place for a joke, but I feel like this thread could use a little silliness.