The association of words with meanings is such a practiced process by most people's minds that it is automatic. Hearing or seeing a word will immediately conjure up the associated mental semantic object, without any choice for the beholder (behearer?).
Normally, that's fine. However, words designated as profanity by consensus tend to be
a) associated with substances or actions that are generally "personal" - masturbation, copulation et al are /generally/ not performed in public, and doing so often generates a similar reaction to the profanity,
b) carry connotations of the worst aspects of the aforementioned topics. Sam and Bob make love suggests an activity that is mutually enjoyed and respectful. Sam fucks Bob imagines a somewhat more one-sided arrangement, with possible violent overtones (depending on the reader).
In other words, profanity as a class will induce an automatic mental imagining of the more unpleasant aspects of activities not usually encountered in a social situation. The visual or aural equivalent of an unpleasant smell being shoved under your nose.
Admittedly, the degree of discomfort varies between individuals, much as it does with smells, but it's still impolite to be so inconsiderate of others' potential feelings.