What do you think an alien visiting our planet would think of people kissing?

Hard to say. It might depend on the alien's own culture regarding physical intimacy between the male and female gender of their species. I would imagine the reaction could run the gamut: horror and revulsion at two people in effect exchanging saliva, curiosity, or even arousal of some kind...if their own tendencies for kissing and carnal relations are similar. ;)

I'd say no, thinking they'd have new genders.

Im thinking it would start all out war *laughs* :D