Now that I've done my private joke, I feel like it would be just a random man that Kaguya fell in love with, or maybe she just wanted to keep the Ootsutsuki blood-line going and grabbed the nearest man, who knows ?
It was most likely another otsutsuki that was kaguya's lover/husband. If it was a human, hamura wouldn't be a pure blood like the last implies. Hagoromo's wife was no doubt a human.