Sassy gay friends are really good at preventing women from making dumb decisions, typically when it involves men and relationships. They excel at this because they have the reasonable intellect of a male combined with the emotional drive of a woman. This bridging gap can also work the reverse way by helping men make better decisions with women.
The Sassy Gay Friend was made popular by the popular Second City Network skit found on YouTube.
Ted: I was almost going to get with Steph, but fortunately my sassy gay friend knew from the drink she ordered that she had herpes and stopped me. Close one!
Alice: I was going to call Karl, but Alejandro, my sassy gay friend, told me to give up.