Aleksei, do you mean they would have sex with the children once and then ask them if they'd like to leave their parents and have sex every day for the rest of their lives? :-)

Anyway, it takes too long for unmodified human children to develop proper minds in order to consent to anything like this. What do you do about pain incurred at the age of a few months? A year?

I'm also bothered that nobody has mentioned non-human animals. Why should cats and chimps and dolphins have to suffer pain and romantic disappointment? The Super Happy People should modify all the higher life forms and completely reshape the ecology.