Blanchflower v. Blanchflower, 150 N.H. 226, is a landmark decision handed down by the New Hampshire Supreme Court which ruled that sexual relations between two females, one of whom is married, does not constitute adultery, because it is not technically sexual intercourse. 
In 2003, David Blanchflower of Hanover, New Hampshire, filed for divorce from his wife Sian Blanchflower on the grounds that she was having an adulterous affair with Ms. Robin Mayer of West Windsor, Vermont. As in most cases of divorce involving alleged adultery, Mr. Blanchflower was seeking an “at fault” ruling against his wife. Mrs. Blanchflower admitted that she was having an affair with Ms. Mayer, but argued that the affair did not constitute adultery under New Hampshire law. 
After a lower court initially sided with Mr. Blanchflower, the New Hampshire Supreme Court ruled in favor of Mrs. Blanchflower and Ms. Mayer, concluding that adultery must involve sexual intercourse under New Hampshire law. In the 3-2 ruling, the majority determined that sexual relations between two female people cannot constitute sexual intercourse and, therefore, the affair was not adultery. The dissenters objected to the majority’s use of the 1961 edition of Webster's Third New International Dictionary as support for their definition of sexual intercourse as between a man and a woman. 
Whether any of the parties tried to escalate the case to federal venue (SCOTUS) is unknown.
Reactions to the ruling were mixed, with some gay-rights groups condemning the ruling as not legally recognizing sex between people of the same gender. Other gay-rights groups applauded the ruling as a victory under the law for one gay couple after many years of discrimination.