The occurrence of one's penis becoming erect as a result of homo-erotic touching/playing, thinking of men engaging in homo-erotic acts, or viewing such homo-erotic acts.
Hershman got a gayboner when Futch gooch-grabbed him after drinking some Arbor Mist wine coolers.
After watching Grey's Anatomy, Ballen had to run to the bathroom to relieve himself of his gayboner.