Bonjour Carl,

Here, you cannot have Le Plus-que-Parfait as it is referring to a situation that happened (i.e. a real situation). Here, we're not talking about a concrete situation but a "hypothetical" one (with a hypothetical couple). Therefore, Le Conditionnel is correct.

You could come across examples as this one with Le Présent though. But, in this case, it would refer to "Le Présent de vérité générale" because it's depicting a "real" cliché/stereotype.

I hope this is helpful.

Bonne journée !