A consent is not such a consent as is intended by any section of this Sanhita,––

   (a)  if the consent is given by a person under fear of injury, or under a misconception of fact, and if the person doing the act knows, or has reason to believe, that the consent was given in consequence of such fear or misconception; or

   (b)  if the consent is given by a person who, from unsoundness of mind, or intoxication, is unable to understand the nature and consequence of that to which he gives his consent; or

   (c)  unless the contrary appears from the context, if the consent is given by a person who is under twelve years of age.