This was my first thought, as well. But after re-reading the article, it's not clear to me that women have a higher tolerance for pain, just that we require two

This was my first thought, as well. But after re-reading the article, it's not clear to me that women have a higher tolerance for pain, just that we require two types of receptors to be active to feel relief from morphine, and that the "complex" of proteins fluctuates with the estrous cycle. I can say that certain painful acts in the context of my multiparous experience have resulted in a flood of pain relief from somewhere within my person shortly after the onset of pain. Things that I was not aware my body could tolerate became tolerable. All very strange.