There are two separate arguments.
One is that sex needs to be kept out of adverts because it can skew purchasing decisions. A beer should be sold on its merits, not because the advertiser hired a model who gets her tits out. A casino or gambling site will not make men more attractive to women. Drinking coke will not make women more attractive to gardeners. Nudity, so the theory goes, has such a strong effect that it can lead to a suspension of normal rational decision making.
Thats fine in general, but is garbage when the product being sold is sex. Thats the one time it IS in context. Sexual arousal is exactly what is being sold.
The other argument is that Art (capital A) is somehow pure, whereas commercial nudity, page 3, porn etc, is somehow sleazy and unwholesome. There is some sexual content that is filmed in an unappealing way, unflattering lighting, rough camera angles, contorted faces, or the scenes are too short for a man or woman to get their bits out and play for long enough to get some pleasure.
There are several problems with that argument. Some classical sculptures and paintings are definitely horny. To complicate matters further, some of the classical greats were secretly gay and creating male nudes for the pleasure of presumably gay patrons and were less interested in celebrating female nudity in a way that might have aroused a public backlash. Michelangelo created what were publicised as ideal or heroic male nudes, but his female figures were less full on and a bit odd. Have a look at Night. The massive thighs. Male? The muscular arms. Male? The glued on breasts. The owl between the legs (where the sun does not shine). It just doesnt compare to the male nudes.
100swallows blog
Where was I? Oh yes, some female nudes were produced by gay artists with no sexual interest in the subject. But some classical female nudes, statues and paintings ARE horny, the argument that they are "pure" does not stand up to scrutiny, and they are just a way of legitimising middle class access to soft porn while locking out the rest of us. So much more respectable to say one is going to an art gallery or reading a expensive coffee table book, than reading page 3 or flicking through Mayfair*. You can only do one with the vicar.
Paradoxically Ofcom surveys found that middle aged women found soft, gentle extended sex scenes with nice furnishings and mood lighting more acceptable harsh lighting and functional surroundings, because it is more romantic and makes it look less as if the woman is being presented for male gratification.
But that doesnt mean they really approved of long or explicit sex scenes, people who were uncomfortable with them prefer ones that make the dramatic point (the couple are in a relationship) and move swiftly on. They dont really want a long scene that could cause trouser warming.
But the Artsy crowd do. They "have to emphasise the depth of the relationship" or "intensity" or some other bollocks.
Being clever with words they get away with it.
Its hypocrisy pure and simple.
* Note for younger readers. Mayfair is/was a thing called a magazine, sort of like a website printed out in hard copy, with pictures.