The main reason for kissing is straightforward.
Yasuke is supposed to be close to the person (always girls) who have his soul, due to a curse. If he stays away from that person for a long time, he will die and will have to be revived by a kiss. The person who has his soul changes many times during the series. This leads to many kisses.
Chars: Yasuke x (Many girls - Amaterasu, Akari, the Goddess of Misfortune etc)
Kiss Type: Direct
Effect: Revive
Example: http://www.mangahere.co/manga/maga_tsuki/c001/14.html
Read the series to find many more examples of this.
EDIT:
Found a gender changing kiss
http://www.mangahere.co/manga/maga_tsuki/c033/22.html
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