How come I can't get turned on when I want to be but..its easy when I don't want to be

i guess you could say its kind of like sleep. its easier to fall asleep when your not trying to sleep (thinking about it too much). this actually a problem, for girls or even myself and my health?

i was at a strip club, one time and I had two girls giving me a private fully nude lap dance. I'm a straight guy and have a girlfriend but I was able to keep from getting turned on, trust me, these two women were gorgeous (for strippers) but I was able to stay turned off.

why you ask, because, you know how embarrassing it been if I did

so why is this?