One psychological explanation for the near-ubiquity of the incest taboo is the Westermark effect, which holds that children develop sexual aversion to people with whom they cohabitate--regardless of consanguinity--from birth to about age six. Demographic studies support this hypothesis, but the phenomenon is not all that well understood. Nevertheless, most humans are strongly repelled by the idea of sexual contact with first-degree relatives.
Sociologists argue that the homosexuality taboo has its roots in the reliance of primitive human groups on reproductive viability. In a modern context, however, homophobia seems largely motivated by personal disgust. Straight homophobes find the idea of coupling with someone of the same sex unappealing, and conclude that anyone who enjoys it must be a pervert (quoth my brother, "Why would anyone want to do 'that'?!"). Gay or bisexual homophobes internalize pressures generated by social/religious/legal prohibitions as well as homophobic peers, and develop a reaction formation to avoid confronting what they see as an undesirable trait in themselves.
As a society, we have begun to accept that 'I find it gross' is not an ethically sound argument for the persecution of those different from ourselves. When the Supreme Court of the United States struck down sodomy laws in Lawrence v. Texas (2003), some commentators feared that the ruling would lead to the legitimization other sexual relations hateful to their eyes. Attempts to overturn polygamy and incest laws have subsequently failed--in the latter case, on the basis of potential harm to unborn offspring. I find that argument startling in a culture so gun-shy about anything resembling eugenics.
Non-reproductive incest between consenting adults is a victimless crime, but it can put someone in jail for up to 20 years in parts of United States. Even inbreeding, while unwise and unfortunate, is effectively little different from non-incestuous reproduction by persons carrying genetic disorders. Image the furor that would result from the passage of a law forbidding marriage or sex between carriers of cystic fibrosis!
Justice means justice for everyone, regardless of how much we revile them or their actions. That is why we permit the Westboro Baptist Church to spew hatred in front of mourners, as little as we may like their message. A free and just society should allow sex between consenting adults, including homosexuality, non-monogamy, prostitution and yes, incest. The price of admission is dealing with other people's personal choices--even if we cannot imagine why anyone would want to do 'that', even if we think it is unwise, and even if we find it gross.