Lots of factors but I think the main ones were a) the kids of the WWII vets were now teenagers/early adults b) England was horribly affected by WWII, it took almost until the time this movie was made (late 1963/early 64) for it to recover but most of all c) birth control pills became readily available in the UK a few years before and were fairly common by the time this was made. Here's a snippet from the Wikipedia article about The Pill, bold mine:
In October 1961, at the recommendation of the Medical Advisory Council of its CIFC, the FPA added Searle's Conovid to its Approved List of Contraceptives. On December 4, 1961, Enoch Powell, then Minister of Health, announced that the oral contraceptive pill Conovid could be prescribed through the NHS at a subsidized price of 2 shillings per month
It's the same story in the US, birth control pills were approved by the FDA in 1960, by 1962 they were used by 2 million + women. Of course, as this movie shows guys don't use condoms, women don't use birth control, babies or abortions result.
That makes complete sense. It's hard to imagine how many changes were taking place during that time. Having gone through the war and its aftermath must have really changed a lot of people's values and their outlook on life. The pill would have been a game changer I imagine.
Woman, man! That's the way it should be Tarzan. [Tarzan and his mate]