Putnam insists that he owns Esther as his handmaid, so he has a right to do what he wants with her, despite the rape being outside of the monthly ceremony. But Esther wasn’t yet assigned to his house when he forced himself on her. And, while handmaid rape, ceremonial and otherwise, is rampant in the series, this action is too much for other commanders, who vote to execute Putnam.
Source: https://www.distractify.com/p/uterus-harvesting-the-handmaids-tale