Actually, with regards to Dean, it's a little of both.
Dean Thomas's background (Chamber of Secrets)
Anybody who has read both the American and British versions of 'Philosopher's Stone' will notice that Dean Thomas's appearance is not mentioned in the British book, whereas in the American one there is a line describing him (in the chapter 'The Sorting Hat').
This was an editorial cut in the British version; my editor thought that chapter was too long and pruned everything that he thought was surplus to requirements. When it came to the casting on the film version of 'Philosopher's Stone', however, I told the director, Chris, that Dean was a black Londoner. In fact, I think Chris was slightly taken aback by the amount of information I had on this peripheral character. I had a lot of background on Dean, though I had never found the right place to use it. His story was included in an early draft of 'Chamber of Secrets' but then cut by me, because it felt like an unnecessary digression. Now I don't think his history will ever make it into the books.
Dean is from what he always thought was a pure Muggle background. He has been raised by his mother and his stepfather; his father walked out on the family when Dean was very young. He has a very happy home life, with a number of half-brothers and sisters.
Naturally when the letter came from Hogwarts Dean's mother wondered whether his father might have been a wizard, but nobody has ever discovered the truth: that Dean's father, who had never told his wife what he was because he wanted to protect her, got himself killed by Death Eaters when he refused to join them. The projected story had Dean discovering all this during his school career. I suppose in some ways I sacrificed Dean's voyage of discovery for Neville's, which is more important to the central plot.
I think its probably safe to say that Dean's biological parents weren't married in a Wizarding ceremony; however, assuming that there's "no" way to divorce in the Wizarding world, it's entirely possible that if he were a pureblood, Dean's father wouldn't think of getting a divorce.
Or he could have known he was being pursued, and fled as fast as he could to keep them away from any sort of attention. The Death Eaters probably didn't realise he'd married a Muggle or they wouldn't be soliciting him so aggressively.