PARIS: Fine, you have four other acts to choose from. Take your pick.
TRISTIN: Yeah, well Summer’s in Act 1, Beth and Jessica are in Act 2, Kate’s in Act 3, and uh, Claire, Kathy, and Mary are in Act 4. So this is the only one free of ex-girlfriends.
Technically yes, but Tristan has been on a date with Paris and kissed her goodnight, and he kissed Rory at Madeline’s party. Louise and Madeline have expressed plenty of interest, and as they are meant to date a new boy every week, it’s not very believable that Tristan wouldn’t have been their boyfriend at some stage, at least briefly.
Note that this is another example of a Beth as an ex-girlfriend – Dean has an ex-girlfriend of this name back in Chicago, who went on to date his cousin. It seems odd to me that Tristan teased Rory by calling her Mary, when he already had an ex-girlfriend named Mary – unless he went out with her after that.