Real simple - married pawns will not accept romances. Can someone please do this?
Puffins' (https://ludeon.com/forums/index.php?action=profile;u=62203) Romance Diversified (https://ludeon.com/forums/index.php?topic=27288.msg276226#msg276226) mod adds several character traits, and among them is faithful, where a colonist will never cheat on their partner.