I would kind of feel like you might not be able to concentrate on either the game or the head fully, so you wouldn't do that well in playing the game, or you wouldn't enjoy the head, one or the other. Once, I was giving my hubby head while he was on the phone ordering pizza. When the guy asked for directions to our house and he had to think about something, he went soft until he put the phone down again. I thought that was priceless, almost as though he had to choose between thinking and fucking.
