Honeymoon rhinitis is a condition in which the sufferer experiences nasal congestion during sexual intercourse.[1]
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 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.[2]
A phenomenon presumably related to 'honeymoon rhinitis' is the occurrence of nasal congestion as a reported side effect of Viagra use.[3]