So I was sitting on a bus the other day and a really sexy looking Thai gal got on board and sat down next to me. She looked at me and smiled and I thought it might be my lucky day. Then I thought “Oh my God, don’t get an erection; don’t get an erection.”

Unfortunately?  WTF? Why disappointed? Was her cock not big enough?  Ever get a blow job from a Thai ladyboy .... the best!

luisn10 you are just like me, trying to analyze the joke.  
It's a joke. To be funny he has to be in the position of being disappointed of not getting an unwelcomed public erection.
Disappointedly the other party did.