The ones who were in arranged marriages were Jack, Mark and Will.

Jack married Gab

mark married Amy

Will married Katie (It would've been Sofia or Chloe but I rather they didn't marry him because of Will's chaotic personality in the book)

Jack married at 17

Mark married at 18

Will married at 17

Will is the only one who had a child in this marriage.

I'll give more info on the marriages.

If anyone's got suggestions of details in these relationships, feel free to say.