It's only degrading if the guy has no respect for the girl. I think it can seem degrading because it's normally done like that in porn, the reason being so that the cameraman gets a better view. If a couple in a long term relationship have oral sex when they're both lying down in bed, it doesn't seem nearly as dirty as when a porn star gets on her knees and gives head to the guy like that. So I think for that reason, and because kneeling is universally considered to be a submissive position, it can be seen as degrading.
The most important sex organ is actually the brain, as human, especially men, are highly visual creatures and are very much affected by what we see. So a man will often be more turned on by oral sex if he can see exactly what the woman is doing. If it's a woman he respects, it's not degrading, just more of a turn on. It can also be a pretty big turn on for the woman. Part of the reason sex is such a turn on is because it's always been seen as a bit of a taboo, and a century ago a woman who actually enjoyed sex might be seen as a bit of a wh*re. Whether we like to admit it or not, the idea of good sex will always be a little bit dirty in our minds. For many women, the idea of something being just a little bit degrading might be a turn on, as long as it's just a play act. If she's doing something which might be seen as degrading with someone she knows loves and respects her, it can be really hot, whereas if it really was degrading, i.e. with some complete jerk, it wouldn't be enjoyable at all. Some women have rape fantasies. It doesn't mean they want to be raped in real life, they just want to pretend with someone they trust enough not to take it too far. It's the same kind of thing, only to a lesser degree.
And I agree with ro-ro; it's much more degrading to have beer poured on you, or your arse slapped in a club, or something like that.