Is Mx. the default title for Male non-phd holders? A journal drop-down menu for selecting a person's title has four options but none of them are Mr.

(I definitely scrolled down enough.)
As a male, non-PhD holder should I select Mx., or is this a mistake by the journal?
Mx is a gender-neutral honorific.  It's probably not a typo.

Dr would typically be used if you have a doctoral degree, and Prof if you are employed in a professor-like capacity (it means different things in different countries, but generally any semi-permanent faculty job is reasonably included).  Otherwise, you can decide which of Ms and Mx fits you better.

If you feel that there isn't an option that's appropriate for you, you can take it up with the journal.