Every table needs a primary key to uniquely identify its records. So, how can you uniquely identify the records in the FormerMarriage bridge table?
Does MID work? No, not by itself. If a man has more than one former marriage, then MID can't uniquely differentiate between them.
Similarly, WID will not work as a simple (single field) PK for the table of former marriages.
How about a composite PK of MID and WID? Will they uniquely identify a marriage? Well, you would think that they would. After all, who would be crazy enough to marry the same person more than once? [Side Note: Elizabeth Taylor and Richard Burton were married to each other twice. She was married eight times to seven people. He was married five times to four people.]
To be safe, you could use a composite PK of MID and WID and MarriageDate.
Or, you could just create a new simple PK field (say, FMID) for the FormerMarriage bridge table.