Complex sentence: When two characters achieve S-rank support, they are married, which both changes those character's endings in the epilogue at the end of the game and means that they can produce a child, or in a few cases, two.
Split into simpler sentences: When two characters achieve S-rank support, they are married. Marriage changes both those character's endings in the epilogue at the end of the game and they can produce a child, or in a few cases, two.