No one cares how an incestuous person gets there rocks off, they care that the implication involves reduced genetic variance, and increased risk of birth defects in resulting children. The usual response from a person playing devils advocate is, "Well what if its not sex that could produce a child?" The answer to that is that all forms of incest are looked upon as aberrant behavior to make doubly sure that people don't have kids through incest. The only feasible way to make sure that child producing incest is ALWAYS frowned upon is to frown upon ALL incest.
You could argue that this is foolish, simply people sticking to a primitive, animalistic perspective instead of looking at something deeper. Ponder this foolishness as you go and blow up people from a different tribe in a video game, go watch a movie about killing people in a different tribe on TV, watch an advertisement filled with boobies, then cook up a big juicy steak while listening to a song talking about some social process that will eventually lead to sex.
Also, you talked about incest at the family dinner table? I'm not accusing you of getting weird ideas involving your family, I'm accusing your entire family of having flipping WEIRD dinner conversations. My family always talked about our respective days, personally.