The U.S. Supreme Court entered the 21st century in 2003. In Lawrence v. Texas, 123 S.Ct. 2472 (2003), the court struck down a Texas law prohibiting "deviate sexual intercourse" (in this case, sodomy) between two persons of the same sex. The Supreme Court overruled its own opinion in Bowers v. Hardwick, 486 U.S. 186 (1986), which upheld a similar law in Georgia. The Supreme Court held that the Due Process Clause of the Fourteenth Amendment protects the decisions of individuals about intimate physical relationships from intrusion by the state, and that the protection extends to unmarried as well as married persons.
The legal effect of Lawrence v. Texas was to render all sodomy laws unconstitutional, including those directed at heterosexual couples. Of course, the laws of the states that continue to prohibit sodomy will stay on the books until they are challenged in court or repealed by the Legislature. If you live in one of those states and are prosecuted for sodomy, however, you should be able to get your case dismissed simply by raising a challenge under Lawrence.
Although it does not address such laws directly, Lawrence also calls into question all restrictions on consensual adult sexual activity, such as laws prohibiting cohabitation and fornication. If you and your partner are over 18 and neither of you is married to someone else, you can consider yourself fairly safe from criminal prosecution for your private, consensual sexual activity, even if your state still has laws on the books relating to cohabitation and fornication.
Adultery, however, is still illegal in many states. These laws may not be affected by the Lawrence v. Texas decision, because of the special public policies involved (for example, support of marriage and protection of children). Adultery laws may still find their way into divorce, alimony, and custody proceedings, or they may be invoked in cases involving property disputes.
Fornication: Voluntary sexual intercourse between unmarried people of the opposite sex. If one of the people is still married, this is generally considered to be adultery, not fornication. In many states, adultery is technically a crime, but rarely is anyone prosecuted for it.
Cohabitation: Two people living together in an intimate sexual relationship, without being married. Some courts, especially those in states where fornication is illegal, describe a cohabitation relationship as meretricious, meaning "of an unlawful sexual nature."
Sodomy: Generally refers to oral or anal sex. Many states call it an unnatural act or a "crime against nature." Some states prohibit all sodomy, while others prohibit only specific acts.More Information on Sex Laws
To find out more about laws (and penalties) regarding cohabitation, fornication, or adultery, including laws that may affect alimony or division of marital property if you're living with someone while getting a divorce, check your state laws. Another useful resource is the National Gay and Lesbian Task Force (NGLTF), which keeps track of sex laws affecting both heterosexuals and homosexuals. The American Civil Liberties Union also monitors sex laws.