Is it a sin for me to have my daughter under birth control? She is mentally and physically handicapped. She is unable to tell someone no, and if she were to get pregnant, it would kill her.
If your daughter is disabled mentally and physically to the point that she is unable to give consent to sexual relations, then anyone who had relations with her, given those conditions, would be raping her. If you fear that your daughter could be raped, then do you really want to entrust her physical safety to a drug that won’t prevent rape and that has the potential for dangerous side effects – just read the warnings on the inserts included with the medication – that could further compromise the health of someone who is already mentally and physically disabled?
Theoretically, someone who is in danger of rape could take birth control medication to prevent pregnancy in case rape occurs. This kind of situation usually occurs with self-sufficient women who must live and work in areas where there is a likely danger of assault. But, in your daughter’s case, she does not need to live under such conditions and it is your responsibility to protect her from people who would take advantage of her. If you can sufficiently protect her from this kind of harm, then there is no need to give her a drug that could further endanger her already fragile health.
For more information, I recommend consulting with the National Catholic Bioethics Center.