Your argument is that the couple tries to prevent such a baby. Which is fine, but it's not an argument for something being morally wrong. How do brothers feel about their sisters when they menstruate? It all depends on the person. Thus rationally we need to prevent pregnancies from such a sexual encounter. Still have a question? Not everyone is going to share your view of what happened.
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Real brother and sister having sex
If consenting adults are having fun, no problem, would be anxious and have the fetus screened if there were to be offspring. The incest taboo, which is universal to all cultures, presumably evolved because the gene pool is at risk when siblings mate. Your argument is that the couple tries to prevent such a baby. Answered Aug 17, Sex between two known persons is not just casual intercourse, it may be more than that. Did you ever wonder why Brandon was pushed out the window in Game of Thrones? Heinlein wrote a series of books which looked into the subject of incest, or at least indirectly addressed it.