When I see a couple of kids

And guess he's fucking her and she's

Taking pills or wearing a diaphragm,

I know this is paradise

"High Windows" was first published in a collection in 1974. In 1960, the birth control pill was made available for contraceptive use. So while Larkin was writing this poem, there was a new culture of sexual liberation flowing through the younger generation, reflected in much that is familiar about the 1960s and early 70s. Larkin views this new freedom to explore sexuality as a "paradise" compared to his own youth, when there was no such highly effective means of preventing pregnancies.

Of course, in this envy, he sees only the need to prevent pregnancies in casual sexual relationships, and this ideology shifted greatly in the 1980s. But this poem predates this new era of knowledge and reflects a common thought of its day: the pill provides sexual freedom, and the younger generation is free to enjoy it.

everyone young going down the long slide

The narrator thinks of this freedom as everyone being on the "long slide" in life. Linked to carefree images of youth, he sees these sexual freedoms as being carefree and joyous. It is also worth noting that he thinks "everyone" is on the long slide—certainly a metaphor—reflecting his envy of their advantageous position. It almost reflects the teenage attitude often heard by parents: "But everyone is doing it."

I wonder if Anyone looked at me, forty years back, And thought, That'll be the life;

Larkin pauses here with a new consideration. When he was young, was he oblivious to some middle-aged man looking at him in envy, jealous of his freedoms? Larkin considers that his generation experienced more religious freedoms than the one before. He was allowed his own opinions of the priest. He doesn't recall being nervous about hell. So maybe this was his own "long slide."

And beyond it, the deep blue air, that shows Nothing, and is nowhere, and is endless.

Where do all of these youthful freedoms lead? Eventually, all teenagers must pass through the same "high window" of adulthood, and in it, there is nothing. It leads nowhere. This sense of a meaningless purpose of midlife seems endless, so perhaps the freedoms of the youth don't matter so much. They simply create an illusion of freedom in marking time toward the endless yet meaningless span of life in front of them.