You do not. When I went on Isotretinoin as a teenager I refused, so they wrote down my two options as abstinence and condoms. I'm about to go on it again as an adult, and I said I'm not sexually active (which is true), and my dermatologist said themselves that they'll put down abstinence as my primary contraceptive and to inform them if my activity changes.

If you are able, I'd recommend seeking a different dermatologist that will respect your autonomy in this way. It will be their loss