> In the Netherlands from what I've seen (at least around Amsterdam) it's almost always forbidden for houses to be rented out to a group of flatmates (e.g. students), some people go so far as to fake relationships to imply they're a couple instead.

The landlord believes that their property and their relationships with the neighbours of the property will be less likely to destroyed by letting the property to older/respected/settled down members of society. Common practice in most of UK as well.