Gay marriage is illegal in the State of Wisconsin. It has been made illegal both by statutes and by a constitutional amendment passed in 2006.
Although people of the same sex are not allowed to marry in Wisconsin, the state does allow domestic partnerships. These partnerships (unlike the ones allowed here in Washington where I live) are explicitly meant for same sex partners. People of the opposite sex may not apply for partnership status.
The law allowing these partnerships was just passed earlier this year.
So, while gay marriage is not legal in the state, Wisconsin is more permissive than some states since it does allow these partnerships.