Are same-sex civil unions legal in Montana?

No, gay marriages are banned, and unrecognized in Montana.

Montana Initiative 96 (2004) passed on 2004 November 2, in the 2004 general elections with 67% of the vote, making it so that only marriage between a man and a woman was recognized in the state

Montana Annotated Code 40-1-401 (1) (d) states that marriages between people of the same sex are prohibited. Further 40-1-403 defines marriage as being between a man and a woman, with no other types of marriage recognized.