natural family planning

I'm 20 and tbh my religion says I'm not supposed to be having sex until marriage but urges are urges! And my boyfriend and I have been having sex for a couple of months now we know it's wrong and even tho we're already doing something wrong we don't want to add to it and we don't use any type of birth control... because it is also against our religion so we just go with withdrawal (which we know is also a form of birth control, but one we can live with)