A sexual slur is a term of disparagement used to refer to members of a given sexual minority, gender, sex, or sexual orientation in a derogatory or pejorative manner. The motivation for using a sexual slur is often sexism and/or bias against a sexual orientation or practice.
Many sexual slurs are used in a wide range of contexts outside of their primary meaning; for example, gay may be used as a term of disparagement to refer to any person or thing the speaker thinks is worthy of contempt, sometimes with absurd results (e.g. "Those sneakers are gay"). Used in this way, the slur conveys contempt by associating its target with an (implicitly despised) sexual minority.
Words which at one point had predominantly positive connotations and came from the minority groups themselves are often eventually used against them as slurs. It is also not uncommon for members of minority groups to adopt an originally disparaging term and use it in a non-disparaging way toward themselves, a cultural process known as reappropriation. Reappropriated words may still be extremely offensive if it is used by members of the majority group (and some will find its use offensive regardless of who does it). The words nigger, Queer, slut, tramp and whore are examples.
Some of the slurs used against women are also used against men, but with the addition of the qualifier man. So slut becomes man slut, and whore becomes man whore. A large fraction of slurs against gay men compare them to women. That is, describing a man as in some way like a woman is considered an offensive slur.
University of New England researcher David Plummer argues that anti-gay terms that often connote effeminate behavior are also used when young boys betray "peer group solidarity" or fail to, "measure up to 'hegemonic' boys' standards rather than necessarily being 'feminine'." Thus, he argues, "homophobia has its early roots in boyhood 'otherness'—specifically in being different from the collectively authorized expectations of male peers." And thus the ambiguous use of the term faggot, which is used not necessarily to mean homosexual, but simply unmanly, cowardly, effeminate, etc.