Reasons for opposition to pornography , or porn, include religious objections, moral values, feminist concerns, as well as harmful effects, such as pornography addiction and erectile dysfunction. Pornography addiction is not a condition recognized by the DSM-5, the ICD-11, or the DSM-5-TR. Anti-pornography movements have allied disparate social activists in opposition to pornography, from social conservatives to harm reduction advocates. The definition of "porn" varies between countries and movements, and many make distinctions between pornography, which they oppose, and erotica, which they consider acceptable. Sometimes opposition will deem certain forms of pornography more or less harmful, while others draw no such distinctions.

A 2018 Gallup survey reported that 43% of U.S. adults believe that pornography is "morally acceptable", a 7% increase from 2017. From 1975 to 2012, the gender gap in pornography opposition has widened, with more women increasingly opposed to pornography, and men's opposition rate declining faster.

Most world religions hold negative opinions about pornography which stem from a variety of rationales, including concerns about human dignity, modesty, chastity, and other virtues.

In Judaism and Christianity, there are numerous verses in the Bible which are usually cited as condemning fornication and adultery, notably the Sermon on the Mount in Matthew 5:28, which states that "anyone who looks at a woman lustfully has already committed adultery with her in his heart." The Catechism of the Catholic Church explicitly condemns pornography because it "offends against chastity" and "does grave injury to the dignity of its participants", since "each one becomes an object of base pleasure and illicit profit for others".

Islam also forbids adultery and fornication, and various verses of the Quran have been cited as condemning pornography and lustfulness, including Surah An-Nur (24:30–31), which tells Muslim women to "lower their gaze and guard their chastity, and not to reveal their adornments, except what normally appears", and Muslim men to "lower their gaze and guard their chastity. That is purer for them. Surely, Allah is All-Aware of what they do."

Some feminists are opposed to pornography, arguing that it is an industry which exploits women and is complicit in violence against women, both in its production (where they present evidence that abuse and exploitation of women performing in pornography is rampant) and in its consumption (where pornography eroticizes the domination, humiliation, and coercion of women, and reinforces sexual and cultural attitudes that are complicit in rape and sexual harassment). They charge that pornography contributes to the male-centered objectification of women and thus to sexism. Andrea Dworkin was a feminist famously opposed to the pornography industry, and proposed the Antipornography Civil Rights Ordinance in several American cities in the 1980s. In 2015, feminist Gail Dines founded Culture Reframed, which responds to the growing pornography industry by providing education and support for healthy child and youth development.

However, many other feminists are opposed to censorship, and have argued against the introduction of anti-porn legislation in the United States, among them Betty Friedan, Kate Millett, Karen DeCrow, Wendy Kaminer and Jamaica Kincaid. Some sex-positive feminists actively support pornography that depicts female sexuality in a positive way, without objectifying or demeaning women, whereas some other feminists don't see any problem with the industry in its current state, given the subjective nature of perceiving humiliation or aggressiveness in a consensual context as something demeaning or negative.

Religious conservatives commonly oppose pornography, along with a subset of feminists, though their reasoning may differ. Many religious conservatives view pornography as a threat to children. Some conservative Catholics and Protestants oppose pornography because they believe that it encourages non-procreative sex, encourages abortion, and can be connected to the rise of sexually transmitted diseases.