There are fashions in film making as well as any other human activity. In the U.S. those fashions were guided by obscenity prosecutions in the 1960s and 1970s, but very little since. One court decision in the mid-1960s listed the duration of screen time as a guide for sex scenes--Anything goes for less than a second, naked breasts for two seconds, and so forth. That led to the editing absurdities you mentioned--1/2 second of pussy sex, 3/4 second of blowjob, back to 3/4 second pussy sex, on and on.
After 1975 or so there was virtually no prosecution for obscenity, so fashions have been less controlled that way. But fashions come and go. Prior to 1990 anal sex was a rarity; after 2000 an anthropologist from Mars would think it is how Earthlings have sex.
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