If a marriage register is signed with an "X" rather than a signature...

[Honor]

Does this typically indicate illiteracy on the part of the person signing?

[Treetotal]

Yes....it is usually followed by the words "his or her" mark.
Carol

[GR2]

It normally does. However, there may be other reasons. I have come across a man who signed his name on his marriage certificate and on the birth certificates of all his children except the last, on which he made his X mark witnessed by the registrar. I was puzzled by this until the 1911 census showed he had recently become blind.

[Marmalady]

Or it can mean the assumption of illiteracy

The poorer classes were often in awe of authority, so if told by the minister / registrar to "make their mark" they might do so even if they could actually sign their name.

I have a ancestress who signed her name on her marriage cert and ran a grocery shop for many years after being widowed, so was presumably literate. Yet when she registered her mother's death, she made her mark -- can only think the registrar assumed she couldn't write and so told her to do so.

[groom]

I've also read that sometimes if the groom couldn't write his name, but the bride could, she would also just make her mark so as not to appear superior to her husband.

[Treetotal]

I've also read that sometimes if the groom couldn't write his name, but the bride could, she would also just make her mark so as not to appear superior to her husband.

Thankfully that ship sailed a very long time ago 
Carol

[Rosinish]

I think this may have been the case with some "Highland" folks who only spoke Gaelic where the Registrar had little or no Gaelic i.e. the person who would sign wouldn't be able to understand & would have to "make their mark"

May also be the case with someone with a broken arm if that was the hand they wrote with   

Annie

Added,

Or someone who was Deaf & Dumb (I know the term has changed) to "Mute" 

[mgeneas]

I have an ancestor who when told to make his mark wrote a beautiful R. He later became a parish clerk so was certainly literate.

[Rosinish]

Perfect example of "assumption" then?

As said, "Never judge a book by it's cover" 

Annie