After reading DD for a while , I've finally got around to registering on the forums .
In the early chapters I was hoping for a Luke/Rory ending, but as the story has progressed their relationship has lent more towards family/close friends than anything else and somehow it would be weird if they started seeing each other.
Seth and Rory are good together and apart from Rory's departure at the end of the summer, they haven't had any serious problems that they haven't talked to each other about, so their relationship can't really be called 'immature'.
There's always the possibility Seth is Wrong/Joking.