Is there any chance that a muggle marrying a witch or wizard will have a muggle child? Is it 50/50, or will all descendants of wizards automatically be magical?
There can be muggle offspring from a wizard-muggle couple, but such cases are rare. So, the probability isn't 50%. The magic gene dominates in most of cases.
Those muggle children are called Squibs. From the linked Harry Potter Wikia page:
A Squib, also known as a wizard-born, is a non-magical person who is born to at least one magical parent. Squibs are, in essence, 'wizard-born Muggles.' They are rare and are looked upon with a degree of disdain by some witches and wizards, particularly pure-bloods. Muggle-born witches and wizards are descended from Squibs who married Muggles; the magical ability unexpectedly resurfaces after many generations.
Hermione is a descendent of at least one Squib.
Having a magical parent does not guarantee magical abilities.
Not only is there a term for the Muggle children of a magical and non-magical parent (Squib), but there are several examples of such in the canon.
Gilderoy Lockhart. Quoting from the “Early Life” section of his Pottermore entry:
Born to a witch mother and a Muggle father, with two older sisters, Gilderoy Lockhart was the only one of his parents’ three children to show magical ability.
Dolores Umbridge. Quoting from her Pottermore entry:
Dolores Jane Umbridge was the eldest child and only daughter of Orford Umbridge, a wizard, and Ellen Cracknell, a Muggle, who also had a Squib son.
(And the award for worst older sister ever goes to…)
We can also construct an existence proof for Squibs based on what we know about magic. Shortly after publishing Deathly Hallows, JK Rowling said in an interview that Muggle-borns must have a magical ancestor:
How exactly do Muggle-borns receive magical ability?
Muggle-borns will have a witch or wizard somewhere on their family tree, in some cases many, many generations back. The gene re-surfaces in some unexpected places.
It follows that every Muggle-born must also have a Squib somewhere in their family tree: the non-magical ancestor immediately below the last person to display magic in their family tree.