First is an ordinal number. If you came in first in a race, somebody must have come in second. Otherwise, it wasn't a race and nobody came in first or second. If you were awarded first honor, somebody must have been awarded the second honor. Otherwise, what they gave you is just an honor. You celebrate a first birthday on the assumption that there are going to be more of them in the future. Otherwise, it's just a birthday. Same thing's true with anniversaries. You celebrate a first anniversary presuming you will be celebrating a second one the next year, and maybe even a golden one forty-nine years later.

So when you talk about a first kiss, you're obviously assuming that there's going to be another one. You can't say, "we had our first kiss" if it only happened once, especially if there is no indication that it's going to happen again. You just say "we kissed." Or you say it like Cookie does and say it was the first and last.

;)