In her 2007 article "Eating Snot - Socially Unacceptable but Common: Why?", Spanish researcher Maria Jesus Portalatin posed the question of why french kissing is considered less gross than eating one's own snot:
As I have already observed, I found hardly any articles written about 'snot eaters'. So I resorted to asking children and adults directly about this 'transgressional' behaviour. I questioned ten adults, aged twenty to sixty, five men and five women. When asked, 'Do you pick your nose and eat the snot?' all the adults emphatically answered that they did not, showing aversion at the idea. However, when asked, 'Do you kiss your partner introducing your tongue into his/her mouth?' the answer was absolutely affirmative and it was accompanied by positive remarks. Clearly these responses relate to what are considered to be appropriate answers in relation to socially acceptable norms. Isn't consuming another person's saliva more disgusting than picking one's own nose and eating the snot? I wonder.
She seems to imply that french kissing is actually grosser than snot-eating.
Later, in the same article, she explains the appeal of snot-eating:
It should not be forgotten that among human preferences concerning food texture, crunchiness is highly significant, and as regards flavours, salty and sweet tasted are the favourite ones. It so happens that dry nasal exudations possess both characteristics, as well as proteins and traces of lipids.
Her article can be found in the book Consuming the Inedible: Neglected Dimensions of Food Choice
, which you can read on loan from archive.org