Last April, Nelson reversed his position and became the 51st U.S. senator to support marriage equality.
From Huffington Post:
There are now just two Democratic senators -- Joe Manchin (D-W.Va.) and Mark Pryor (D-Ark.) -- who are not co-sponsors of the legislation, which would bar workplace discrimination on the basis of sexual orientation and gender identity. It is already illegal to discriminate on the basis of race, color, sex, nationality, religion, age or disability.