How are pelvic pain and fertility related?

Similar causes. Many of the disease processes that cause pelvic pain can also interfere with normal fertility. Examples are endometriosis, pelvic adhesions, and fibroids. If you would like to become pregnant and suffer from pelvic pain, have it evaluated in a timely manner. For example, endometriosis gets worse over time if measures are not taken to prevent it from progressing.
It's very complicate. They are conditions that can be related to each other or not. Certain causes of pain can cause infertility. But they are not necessarily related. Really l, a thorough explanation is way beyond te scope of 400 characters.