Anterior nosebleeds make up more than 90% of all nosebleeds. The bleeding usually originates from a blood vessel on the nasal septum. Anterior nosebleeds are usually easy to control, either by measures that can be performed at home or by a doctor.
It is little known, but your penis and inside nose tissues are made up of the same sort of thing.
Some people sneeze when they have sex and sometimes decongestants can interfere with erections or delayed orgasm.
Thus, when watching porn:
Your heart rate will have greatly increased leading to a rise in blood pressure within your body. Also you will be tensing your neck muscles which may be slightly obstructing the venous drainage from your head.
Both of these will lead to an increase in blood pressure within the blood vessels lining the nose and sinuses. If they are prone to rupturing then this is the time they will do it.