Mont. Code Ann. § 45-5-708

In a prosecution for an offense under 45-5-702, 45-5-703, 45-5-705, 45-5-706, or 45-5-711 or a civil action under 27-1-755, evidence concerning a specific instance of the victim's past sexual behavior or reputation or opinion evidence of the victim's past sexual behavior is inadmissible unless the evidence is admitted in accordance with 45-5-511 (2) or offered by the prosecution to prove a pattern of human trafficking by the defendant.