It is normal to bleed after sex some of the time, but not all of the time. A woman will bleed the first few times she has sex because the skin of her hymen is torn. She will bleed when she has sex just before or just after her menstrual period because there is blood left over or ready to flow and the sex pushes it out.
The reason most women bleed if not for the above reasons is that the guy is too rough, does not use enough lubrication, or she has an infection or cysts that need to be addressed medically with a doctor.
If you bleed every time you have sex, you need to discuss it with your doctor and start using lubrication and get your sexual partner to be more gentle.