I'll say it again
the first kiss is more exciting than the first sex
there's so many opportunities, places, and ways to write the first kiss
it can be cute, funny, romantic, sexy...
but sex?
you can't really have a funny sex scene...
and there's not that many places your characters can have sex without being seen as perverts
in conclusion:
kisses are more versatile.
in the sense of censorship.......