Most Helpful Guys
There is this idea that if a man can make a woman orgasm from oral sex, then she will want to have real sex with him. And that is compounded with the belief that women are more open to the idea of being eaten out on the first date than to have full sex on the first date.
I am not saying that this is true, but some people do hold that belief. I believe it was even a plot point in one of the American Pie movies. Either that, or he just likes giving oral to girls.