Careful reading of Culhwch ac Olwen and the Triads indicates that while a lot is known about Arthur's mother's family, including his uncles and his cousins, we are told absolutely nothing about his father. He is not, in this tradition, Uthyr; he just isn't mentioned. If this was an Anglo-Norman world-view, I would suspect illegitimacy (which is the path the later romances go down) but this is Welsh traditions, which see little details of whether the parties were actually married as somewhat irrelevant. In fact, there are seven forms of marriage, ranging from a feast laid on by the bride's father down to abduction and rape, which leaves a lot of leeway.
My suspicion is that the Welsh sources didn't know, and didn't care about, Arthur's father's family for the reason that his father wasn't a Romano-Brit at all. He has a "foreign" (non-British) name; he has the sort of morals best associated with people like Clovis; he's a poet who hasn't gone through the normal British procedures. My suspicion is that Arthur's father's family was Germanic in origin. After all, most of the best generals in the Roman Army in the 5th century were German stock; Aetius is about the only exception.
Explode if you like, but I'm really more interested in reasoned arguments (preferably with sources) against the idea.
Happy New Year!