This is used to describe a person who is heterosexual for most of their life, but 'openly' homosexual towards the end. Like a lamppost: straight from the bottom up but bent at the top.
J - "did you hear? Jack left his wife!"
T - "oh really! after thirty years!?"
J - "yeah, apparently his gay!"
T - "snap! must be on of those lamppost homosexuals!"
J - "Ha ha, yeah! straight for most of your life..."
J & T - "...but bent in the end!"