swapping saliva

why is it that i can kiss someone, share their toothbrush in emergency situations, taste the pasta sauce before i serve it to him, but not share forks/spoons when we're dining out? is there some stigma to sharing cutlery? does it signify a bridge that we haven't crossed? is there some contamination factor to the dining-out experience? if we share forks while splitting a delicous piece of blackberry shortcake with white chocolate sauce and raspberries, does it reveal that we're comfortable enough with each other to let me borrow his socks when it's cold out (more man sock per square-inch of my foot means more warmth contained per second!) or floss in front of the other or blow our noses right before cuddling? the man in question has no problem drinking out of my glass (in public), so i have to wonder. what's the deal?