Do you realise that kids before puberty don’t have much difference from biological point of view. Sexual organs are not developed or fully developed and no hormones to speak of.
The story of Lego you said… cool you can control parents behaviour… what about peer pressure? Or the idea was to control the parents of a whole town (including Cartoons and TV shows)?
If you like football and your best friend doesn’t like football, what do you do? You beg your best friend to play football… but if they really don’t like football or it is just a “once in a while thing” what you do? Probably find another friend or group of friends that like football. Your best friend likely stays your best friend but now you are happy, your best friend is not annoyed by you asking playing football and you have new friends
Of course you don’t hide or lie to your best friend on what you do and when… otherwise they would be (rightfully) upset but if you share and they are mad that you want to have new friends it’s on them to be unreasonable.
Well here Ethical non monogamy in a nutshell in a context where it is socially acceptable 🤷♂️