If that really is what they meant, they are still regarding it as “good” or “bad” through the lens of monogamy. Free love, polyamory, and all that jazz don’t necessarily bind themselves to the disney interpretation of love “one partner forever and if you don’t, you’ve failed”.
Some people are happier having multiple partner sequentially or in parallel. Some may have one stable partner and at the same time multiple others that come and go while retaining that stable / main / primary partner. Others may move in and out of relationships with the same people simply because they have other definitions of “relationship”. And so much more.
The evaluation of “good” is highly subjective.
Why are people so weird about sex? It’s nothing special. It happens all around the world, probably millions of times per day, we’re all here because of it, it’s natural, and with consent it is probably one of the least harmful things humans can do to each other. People should be given the place and time to engage in sexual activities comfortably, safely, and lovingly. They should be able to get to know their own bodies and explore those of others to learn and have fun.
It’s the parents’ job to create a good environment where the children can feel comfortable talking to them about sex and learning important lessons from them e.g that porn is not a good representation of what happens in a bedroom, how important communication during sexual activities is (verbal or otherwise), how every partner is different, and so on.
Be the good parent.