Why did clytemnestra marry agamemnon if she hated him?

Clytemnestra may not have had a choice, in this I mean her father Tyndareus may have forced her to marry Agamemnon (King of Mycenae): her sister Helen certainly married Menelaus, King of Lacedaemon (and later Sparta) in something like a lottery of suitors. The reason being they were daughters of the King and Queen of Sparta.

Also in myth Clytemnestra was married first to Tantalus, King of Pisa or Broteas, King of Lydia, whom Agamemnon slayed and killed also her infant son with that King making her his "wife" in the sense of spoils of the killing a king.