I really find this difficult to express properly, in terms of, how on one hand the Dutch are one of most liberal societies in the world regarding sexuality, yet on the other are more strict at setting acceptable standards of right and wrong.
It's not even complicated. The keyword is "consent".
As long as all involved parties are fully informed, consenting adults, they can do whatever the hell they want when it comes to sex. Nobody else needs to interfere. That's your "liberal".
Of course, children cannot consent, and a skewed power balance (for example when a 16 y/o is 'in a relationship' with a 35 y/o) makes proper, fully informed consent impossible. That's your "strict".
Again, the keyword is informed consent.