Your first thought was the right one: It stands for Male/Female. In German job offers, the term
(m/w) (männlich/weiblich -> male/female) is used.
This comes from the fact that in German, nouns like job descriptions have both a male and a female form (e.g. Lehrer and Lehrerin for teacher). Since companies want to show that they are open to male and female applicants, but always writing both forms is a waste of time/space/letters/bandwidth, the form
Job Title (m/w) came to be.
The advertisement you read was obviously translated by someone who didn't think of the fact that English does not have any gender distinction in job titles.