For 1,500 years this was the scientific consensus. How could we have continued to believe in the necessity of female orgasm when there must have been all kinds of evidence to the contrary? No one is sure, according to Daniel Bergner. When it comes to the study of female sexuality, scientists have tended to see what they expect, or want, to see, and there are fewer established facts than you would think.
I think to answer this question requires not just looking at the history of sexuality but the history of scientific inquiry generally. Galen did practice experimental anatomy, but he was one of a few, and perhaps this type of experimentation was just not accessible. More generally, most scientists ascribed to the Greek method of thinking in the ideal and rejecting experimentalism out of hand. For example, it took 2000 years to dispel the accepted 'truth' that heavier objects fall faster that light objects. Do this experiment in your house, and you can see how easy it is to show that your pencil and bowling ball hit the ground at the same time. But it took 2000 years to convince educated men of this fact! Why? Because great philosophers had 'proved' otherwise by thoughtful analysis. My guess is that there is something similar afoot here. Physicians probably noticed the orgasm, wondered about why it existed, and came up with what they thought was the most logical answer: Men need an orgasm to release their seed, therefore women do, too. It would have been heretical for a young physician to test this hypothesis once it was settled. I think this is a much more likely explanation than confirmation bias based mainly in misogyny (not that didn't exist, it just doesn't explain the history as well).