Either Zyl is a king of trolls or I am gullible in the extreme. He told me that OVA of Quiet Morita-san had definite proof that Mayu's father was actually unfaithful to Yumi. I downloaded the two episodes that are not available at Crunchy, and he's not in either of them. As far as I'm concerned, Yumi's groundless jealousy is the source of funny hijinks in the anime canon. She is also a little broken in the head. I say that on the basis of sitting in seiza overnight in front of the door just to give the guy a good earful.

UPDATE: OVA (thanks, Zyl).