incest? what do u think?

I've actually just been having a discussion about incest. Well I say "discussion", more of me questioning why someone is so grossed out by the concept of incest and them just saying "it's wrong and creepy" without much of an arguement to back up their claims. They couldn't give me a straight answer to the questions I'd posed, instead just kept emphasising points they'd already mentioned. I see it a lot, so I try to make people rationalise why they think like they do and a majority of the time nobody can answer my questions. Some even get defensive about it.

My theory is that people are taught that taboo things are "not of the norm" whether they're being directly told it isn't or if they learn from their environment. Things we learn from an early age, possibly due to peer pressure, becomes our rationale. We blindly follow these beliefs without actually thinking about it. When questioned, we get defensive because we don't want to hear that we're wrong or to apply some rationale to a belief.