Children engage in erotic play because it teaches them to equate sex with meaningless fun.
Sex plays a very different role in the modern society than it does in ours. To them, motherhood and birth are considered vulgar. Sex is just another activity that people do as a diversion, and is intentionally separated from love.
Erotic play is encouraged among younger children to ensure that they do not ever associate sex with love. In fact, when young children do not engage in the sexual games they are considered abnormal.
"It's just that this little boy seems rather reluctant to join in the ordinary erotic play. I'd noticed it once or twice before. And now again today. He started yelling just now …" (Ch. 3)
If the children do not engage in this type of play, it is assumed that there is something wrong with them. It is very important that everyone fit into their role in society, so they take this very seriously.
All individuals in this society are clones. Most of them are infertile, except for a few who wear Malthusian belts with contraceptives. The “freemartins” can have sex without consequence. Either way, this prevents unwanted babies.
Men and women who have a lot of sex are respected in this society. Consider the description of Helmholtz Watson.
This Escalator-Squash champion, this indefatigable lover (it was said that he had had six hundred and forty different girls in under four years), this admirable committee man and best mixer had realized quite suddenly that sport, women, communal activities were only, so far as he was concerned, second bests. (Ch. 4)
Just as children are considered abnormal when they don’t engage in sexual play, adult men and women are considered dysfunctional and weird if they do not take many casual lovers. There is no such thing as monogamy, which they view as wrong.
In a world where "mother" is one of the most vulgar words imaginable, sex is nothing more than a pastime. People are dependent on mass-consumerism and idle entertainment in an endless pursuit of pleasure with no substance. This keeps the population in check, and ensures that overpopulation will never occur. That's easy when everyone is a clone.