Section 91 of IPC is a proviso to Sections 87, 88 and 89. Sections 87, 88, 89 categorically mention that consent will only condone the act causing harm to the person giving the consent, which will otherwise be an offence. Acts which are offences independently of any harm which they may cause will not be covered by consent given under Sections 87, 88 and 89. Such acts can be: causing miscarriage, public nuisance, offences against public safety, morals etc.
If one considers the example of causing a miscarriage, then it is not only an injury to the woman alone, but an offence against the life of the child as well. Ergo, the mother’s consent will not be considered as the consent of the child and the person causing a miscarriage will not be absolved from prosecution by claiming consent as a general defense. Consent can be given to condone a harm caused to the person concerned, but not an injury caused to someone else, who never consented to it, (Ademma, (1886) ILR 9 Mad 369) consent will not condone a man from criminal prosecution in respect of offences against the State, public health, safety, convenience, decency, morality and the like. Thus obscene publications, indecent exhibition and public nuisance, etc., all such acts are acts which can come under the ambit of Section 91.