A taboo is commonly defined as something that is forbidden by a specific society. There are many taboo topics in America that are not normally discussed. For example, a bride who is noticeably pregnant on her wedding day was historically considered a taboo topic that was quietly whispered about but not officially acknowledged.
Religion can be a taboo topic in America. The United States is generally known as a melting pot of people from all over the world. This wide variety of people bring with them not only their belongings but their religious beliefs. Many faiths that have come to America, including the Islam, Catholicism, Judaism and Hinduism, among many others. These faiths are often parodied in movies and by comedians because of the taboo involved in poking fun at another person's beliefs.
Polygamy is when one person has more than one spouse at the same time. It is also a topic that is not spoken out loud about very much due in real life to its illicit nature. Polygamy was often practiced in the Mormon religion in Utah until it was made illegal in the late 1800s. Polygamy continues to be practiced today by some splinter Mormon groups and other cults, though it is still very much a taboo to practice today--not to mention that it is still illegal in all 50 states.
The topic of abortion can be very emotional for some people. There can be strong opinions on both sides of this issue, and generally no middle ground exists. The topic is frequently written about in newspapers and discussed by politicians. However, it is normally not brought up in casual conversations, and it is taboo to ask women if they have ever had one.
The legal definition of pedofilia includes the molestation of, taking pictures of and exposing one's private parts to a child. These acts are seen as morally reprehensible, and pedofiles are often ostracized by communities. Normally this emotional issue is seen as taboo in a casual conversation or even as part of a comedy act.
Incest, defined as intimate relations between two people who are closely related to each other, is generally not a topic for casual conversation. It is commonly viewed as a morally wrong behavior, and is not only a taboo in America but all across the world. The taboo against incest has a biological underpinning--offspring produced by closely related parents are more prone to genetic abnormalities, so as human societies evolved, those who frowned on incest tended to survive better than those who allowed it, eventually leading to a general taboo against incest.
Negative remarks regarding someone's race can be seen as a taboo topic in the United States. Even though such words or phrases may be a part of pop culture, it is still commonly frowned on to use them in causal conversation.