Sorry I should have clarified, married men and women are not segregated from eating in public. They do have screens sometimes around tables but that is so women can remove their niqabs to eat. The strictly religious might insist on it in their houses, but this hasn't been by experience in any Saudi's house I've been in.
As for singles, they can't mix at all, in public or otherwise, so therefore wouldn't be able to eat together in any case.