Honeymoon rhinitis is a condition in which the sufferer experiences nasal congestion

Nasal congestion

Nasal congestion is the blockage of the nasal passages usually due to membranes lining the nose becoming swollen from inflamed blood vessels. It is also known as nasal blockage, nasal obstruction, blocked nose, stuffy nose, or stuffed up nose.Nasal congestion has many causes and can range from a...

 during sexual intercourse.

The condition appears to be genetically determined, and caused by the presence in the nose of erectile tissue which may become engorged during sexual arousal as a side effect of the signals from the autonomic nervous system

Autonomic nervous system

The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

 that trigger changes in the genitals of both men and women.

A related (but very rare) condition also exists where people sneeze, sometimes uncontrollably, when engaging in or even thinking about sexual activity.

A phenomenon presumably related to 'honeymoon rhinitis' is the occurrence of nasal congestion as a reported side effect of Viagra

Sildenafil

Sildenafil citrate, sold as Viagra, Revatio and under various other trade names, is a drug used to treat erectile dysfunction and pulmonary arterial hypertension . It was originally developed by British scientists and then brought to market by the US-based pharmaceutical company Pfizer...

use.