I think that a good means through which to understanding the nature of Fi in the context of taboos is to contrast its fundamental nature to that of Ti.

My best friend down here in Florida is EII, also a devout Christian, and he and I playfully pick at each other's Super Egos. He doesn't know about Socionics, so he isn't privy to the informational metabolic conflict that is occurring, but what frequently happens is that he will make some Fi statement, usually relating to religion, about what he thinks is right, and I will gently ask him to justify his belief. What invariably follows is a confused, sort of muddled reaction on his part, then a kind of flustered aura of defiance in which he asserts that his position is simply how he feels.

Conversely, I will, in a sort of frazzled, half-serious childlike way, as I often do, make some harsh statement regarding something I don't like, using stiff, rigid logic as the primary justifying element. His response to this is usually a kind of soft Delta Fi rebuke of my bitchy Beta Ti, uh... asseveration: "Peter, why did say that? That's not nice," and my inevitable answer is a narrow, overly linear A + B = C kind of defense that, even though possesses logical coherence and kind of ascetic definitiveness, is obviously too harsh to be considered reasonable.

Taken too far, both Fi and Ti exculpations can result in calamity: Ti through detachment from humanity; Fi through detachment from coherent thought.

But anyway, what I am saying is that, whereas Ti is basically core sentiment justified by logic, Fi is core sentiment justified by some kind of preternatural instinct. I think taboos are essentially a product of Fi, because they amount to something ignoble, reprehensible, etc., adjectives whose continuances aren't justified logically.

(Fe types are aware of taboos, but seem to prefer reading between the lines of statements and actions which violate their core tenets, only enforcing them when they deem it necessary to not ruin the atmosphere/piss on the parade, etc. I find that Fe seems to enjoy it when Ti types come in and bend the rules, as it were, regarding what is and isn't permissible, based on arithmetical computations, as it were.)

Another parenthetical aside: (Tam's Christian friends are all much nicer people than the non-Christians I know, by far.)