But negotiating is a part of doing any business. Lets say your roof starts leaking... you're willing to pay a lot to have it fixed because the consequences will be very bad very very soon... but you still call around, ask a couple of companies/tradesmen for the price, and choose the best price/performance one, while hoping there wont be any heavy rain before they're able to fix it.

If all of them said twice the price, you'd still have to pay... three times the price too. If it's something you're unable to do by yourself, and if not doing it makes it even worse, you basically have no choice. This applies in the same way to job openings.

The no-negotiating theory would just mean that employers would be fighting for women employees if they were indeed cheaper for the same work output, until the competition would bring the prices up to the men levels.