Having sex everyday will not do anything to cause fertility problems later in life. What CAN cause fertility problems is STD's that go untreated. Chlamydia and gonorrhea can, if left untreated, cause infertility in the woman. It is important to ask your physician for regular checks against these two diseases because they often go unnoticed. So other than having a STD like these that doesn't get treated, there isn't anything regarding sex itself that can cause infertility. Take care.