I found it!

Okay, so Tally was thinking when she kissed David how uglies did kiss each other, but it never seemed to count until they were pretty. (Luna, you've seen all this, but still)
David was definitely her first kiss that counted, but I don't think it was her first kiss ever.
From the book:
Uglies did kiss each other, and a lot more, but it always felt as if nothing counted until you are a pretty.
Hah! I rest my case.