So can you ever have sex as many times as they want and never get pregnant?

Yes. If lots of unprotected intercourse does not lead to pregnancy, especially after one year, then you may choose to be evaluated for infertility. In most cases of infertility, there are issues from both partners, ie low quality sperm or low sperm count from the male, and a wide variety of female issues. Call your doctor. But don't count on infertility for birth control until you know for sure.