The BF system allows you to have a Best Friend instead of a husband. One of the special girls from the boy version can become your own closest friend. A Best Friend will even give you a gift on your Birthday! Who wouldn't want more birthday presents? If you want a Best Friend you won't be able to choose to have a male husband later.
To acquire a Best Friend you have to go through the Heart Events for either Keria, Leia, Goddess, or the Witch. When you talk to a girl while having the Love Ring equipped you can see how far along you are with her Love Points. Only the 4 special girls have their Love Points available for view, leftover from the boy version.
After you trigger the Yellow Heart Event, you have to give your chosen Best Friend the Blue Feather that you'll need to get married. The two of you will then have a Friend Ceremony inside your house. Before you can choose a Best Friend you have to complete the standard marriage requirements of finding 60 Sprites, owning the Big Bed, and triggering all 4 heart events.
You also can have a child gifted to you if you are connected to a BF instead of a normal husband. The Harvest Lord will deliver you a random-graphic child when the time is right.