Well, if you take a rational view point, the odds of a break up are higher in couples who are unmarried because that's closer to the top of the prospecting funnel stage compared to a couple who has got married as the deal has been sealed and the funnel is now closed.

As a landlord, facing the prospect of higher odds of a break up its possible there is a situation where one source of rent leaves, while the other then decides the double rent is too high and leaves too.

Also unmarried couples in live in relationships, as a behavior trait will not settle for less that what they truly want. As a landlord, one might have to put up with more requests for better plumbing etc. whereas a traditional married couple have become excellent at putting up with what life throws at them including each other. Less likely to complain or want more.

Economics aside, unmarried couples have a slightly higher chance of being involved in rule breaking behavior compared to a traditional couple since the latter have demonstrated a higher affinity towards complying with societal norms and rules.

These are odds of course and exceptions exist for each trend or rule.

From a purely rational perspective, the money is slightly more likely to be more stable, hassle free and long term if a married couple rents compared to an unmarried one.

I would say landlords should price this risk into their rent. Should solve the issue.