implantable contraceptives
The is already a long-term implantable contraceptive pill; IIRC it has small hole at one end so that the drug slowly leaks out, and into the woman's bloodstream. If it is removed early, fertility generally returns within a few months.
Given this, it could be possible to put a small ball-valve (Silence at the back!) at the end of it, with a NFC powered link to turn the valve turn the contraception on and off; it would remain in its last state when not powered. If the remove can query the state of the valve, then it will reassure both partners about the state of the device. However, the woman, could, if she doesn't want to get pregnant, but her partner does, turn the device back on after sex.
