Good question. Suppose that Mike and Abigail are dating, and they decide to get married. They go to their religious leader or a judge (or equivalent) and the religious leader or judge marries them, that is, the judge marries Mike to Abigail and Abigail to Mike. This is why we say that Mike and Abigail *get married* (to each other) — it’s a type of passive voice (the judge is the actor). Nowadays however, it’s normal to say (without any preposition) that Mike marries Abigail / Abigail marries Mike / they marry each other. This modern usage is also considered correct, and has been gaining popularity for several generations.