Incest isn’t a social taboo in many cultures – it’s forbidden by practically all human cultures. The ban on incest is about as universal as the ban on murder. In some cultures, an exception is made for monarchs, because it’s socially and politically advantageous for monarchs to marry relatives who are also high-ranking. The ‘increased birth defects’ explanation of the incest ban actually doesn’t hold up experimentally – anthropologists have different explanations for the ban. One seems to be that humans are innately repulsed by the thought of incest, as they are innately repulsed by murder.
On the contrary, precepts against homosexuality are by no means universal. Homosexual relationships have been integral to many cultures (e.g. the ancient Greeks, and some cultures in Papua New Guinea). Many species of primates form homosexual relationships (bonobos, for example, are pansexual and engage in group sex with members of both genders), but no primates are incestuous (males often leave the group they travel with as infants to join another group later in life). Humans do not have an innate repulsion to homosexuality, and it is natural to a certain percentage of them. Precepts against homosexuality can probably best be explained as repulsions or pratically-based bans (if men lie with man as with woman, then God will have fewer chosen people) that were imposed upon cultures by a small group of people. The Hebrew moral code, upon which the Christian moral code is based, happened to forbid homosexuality under some circumstances – the modern incarnation of homosexuality was not forseen by the ancient Hebrews. So it has happened that much of the world believes that homosexuality is wrong mostly because they share the moral code of one of the ancient cultures that forbade it.
Incidentally, there is no parallel between incest and homosexuality. The argument made in the topic is just as valid as “It’s okay to be gay, but if you like prepubescent children you’re a freak” or “It’s okay to be gay, but if you have sex with animals you’re a freak”. You can’t just lump together all non-heterosexual non-marital sexual activity as ‘freakish’, and then compare the natural sexual orientation of a small but significant percentage of the population to a practice which is forbidden by nearly all human cultures. Certain sexual practices – homosexuality, pre-marital sex, non-procreative heterosexual intercourse, and so on – are practiced by a large number of people and are not considered repulsive by many of those who don’t practice them, even though they’re forbidden by the Judeo-Christian code. Others, such as incest, bestiality and pedophilia, are universally recognized as repulsive.