It's marital status discrimination because it's banning behavior only for singles. If that weren't discrimination against marital status, then the following scenario wouldn't be discrimination against religion:Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
I own a company, I don't want to hire christians, but I'm not allowed to discriminate based on religion. So instead, I make them sign a contract saying that christians working here aren't allowed to ever sleep, even in their freetime, even in their own bedrooms. If I see evidence that they've been sleeping, like them coming to work well rested, I can fire them for violating the contract.
This effectively prevents them from working here. It's an extreme situation, clearly, but it shows the point. I have discriminated against them because I have made up rules that only apply to them.