Whalers did not read Galen or any other Medieval authorities.
No Bill, but they were treated by doctors that did have indirect reference to Galen & co, and who would have physiological explanations to suit. It was a powerful paradigm that encompassed animal as well as human physiology. People were still being treated by being bled (of excess internal fluids) right up to the 19th C, I believe. They would doubtless ask the doctor what he thought he was doing, and word would get around.

But they knew how eunuchs and oxen were created.
Galen & co. were well aware of the existence of testes and the effects of castration. They just explained these facts in a way we now find unbelievable.

It's fairly hard to believe that the world was thought of as flat, and that the stars were meant to be fixed lights in huge rotating crystal spheres, that made sweet music as they revolved. But everything's straightforward in hindsight. These ideas fitted well with the generally accepted knowledge of their time, and were adopted by all who had an interest (or needed to have an interest - sailors, for instance).