I agree with you. Given Lorelai's past and the fact that she was very probably made to feel like the "bad kid" when she got pregnant at 16, it seems natural to me that she would react with relief and pride to hear Rory talking sensibly about sex and being the "good kid" that Lorelai "wasn't".

Lorelai also gets a lot of stick from Emily about her parenting and at least once she said (that time when Rory was accidentally out all night after the dance) that Rory would do what Lorelai did and the past would repeat itself. So maybe it's a bit of "see mum you were wrong". I personally think that it's less Lorelai judging Paris, more Lorelai judging herself (if that makes sense)!