After having sex, can sex be the reason for late period?

Pregnancy secondary. to sex is a reason. Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (i.e. Pcos). #1 reason is pregnancy.
Late Periods. The most common reason for a late period is pregnancy. Sexual activity does not have any power to change the menstrual cycle. So, the answer to your question is no.