Honeymoon rhinitis is a condition in which the sufferer experiences nasal congestion 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 that trigger changes in the genitals of both men and women.
A related condition called sexually induced sneezing 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 use.
References
Honeymoon rhinitis Wikipedia(Text) CC BY-SA