This is the true heart of the lawyer's case: that Grandjean genuinely believed--on whatever basis--that they were able and authorized to take on the role of husband in a marriage. Although the lawyer brings in anatomy as part of the basis for that belief, it isn't in fact necessary for the argument to prevail. This is where the specific charge (profanation of the sacrament of marriage) is key to Grandjean's acquittal, because it's the only one that can be refuted.