Someone who is heterosexual. Names comes from New York City's famous, and affluent avenue that runs north on the eastern side of Manhattan. Manhattan's east side is considered to be too stuffy, snooty, and not chic for gay men, and the fact that Madison Avenue runs "straight up" Manhattan has made this a very popular term.
"There is no way and how you can convince me that Bob is a Mcgreevey case. That boy is 100% Madison Avenue!"