Nobody is denying that, statistically speaking, there are disparities between the physical capabilities of men and women. So, your concern over usage seems a bit misplaced, especially when feminists themselve use the term 'equal' in the normative rather than the descriptive sense. Nobody is denying that there will be jobs that suit men better than women (heavy lifting, for instance). The vast majority of jobs out there, however, don't generally favor one sex or another. Yet, it is still the case that in the United States, women receive roughly 3/4 the pay of a man for doing the same job. I'm sure we all agree that such disparities in pay are unjust. I'm sure we all agree that agitating for equal pay for equal work is justified.