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, very famous actors of your grandparents' generation, 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.