As far as I can tell it has nothing to do with marital status, ie married or single. It has to do if the person is crazy. You can hire or not hire people based on personality and fit with the company, so I'm not sure why the spouses personality and their relationship fit for the company would be off limits.
It obviously does discriminate against people by marital status, because if you're not married, you're not liable to be rejected based on the spouse-dinner filter.
It's a little like suggesting that you're not filtering based on religion if you ban yarmulkes.
I'd say its closer to finding out the single guy is in a gang and since he spends time with crazy people you don't want him. But I guess your point is at least a bit valid. That being said, it seems only a few states have a problem with using married status in hiring.
