Wow! I can't even imagine anyone having an issue with someone kissing their horse. I am totally fine with anyone being kind and gentle to any of my critters. I mean, I would prefer they ask before feeding them anything but giving them a kiss? Go for it, as long as it isn't freaky and the horse doesn't mind.
I have to admit I am sometimes taken back by people that are so " IT'S MINE! DON'T TOUCH!" I wouldn't ever want my horses to have less attention simply because I am over possessive.