Originally Posted by bibabydoll
Webmd.com is your friend. Read the sections on Natural Family Planning and Fertility Awareness Method if you really must use a secondary birthcontrol method besides a condom. She will have to keep track of her menstrual cycle, temperature and mucus coming out of her vagina if you really want to know when she is least fertile and even then its not 100%
Beat me to the punch.
My lady and I rely on birth control, haven't used a condom in years (I have when with her friends, but for other obvious concerns) and don't really give any thought to when we fuck, we just do it when we feel like it, I almost always cum inside and so far we've had no problems (over five years of doing this).
That's not to say you won't in the same situation. My brother and HIS girlfriend did the same, she had three miscarriages and an abortion (serious developmental problems and health concerns) before they split up.
If for religious or other reasons birth control is out of the question, bibabydoll hit the nail on the head; your basic methods of Fertility Awareness are BBT (basal body temperature, which my Catholic buddy and his wife use, and they have two boys they didn't plan for), cervical mucus, cervical position, and combinations of these methods. Check online, there are a lot of resources.