What happens if a 20 year old has sex with a 15 year old. Would this be sexual assault? If convicted would the 20 year old have to register as a sex offender?

Under 45-5-501 Definitions, the term "With out consent" is defined as sexual conduct with anyone who is less than 16 years of age.

MT Code 45-5-503. Sexual intercourse without consent:

(3) (a) If the victim is less than 16 years old and the offender is 3 or more years older than the victim or if the offender inflicts bodily injury upon anyone in the course of committing sexual intercourse without consent, the offender shall be punished by life imprisonment or by imprisonment in the state prison for a term of not less than 4 years or more than 100 years and may be fined not more than $50,000, except as provided in 46-18-219 and 46-18-222.

If found guilty of violating the law above, this would be a qualifying offense for Sex Offender registrations.
Source: Dept of Justice - Montana

MT Qualifying Sexual Offenses

Individuals convicted of one or more of the following offenses are designated as sexual offenders
and are required to register with Montana's Sexual or Violent Offender Registration Unit.

• Aggravated Kidnapping – MCA 45-5-303
• Aggravated Promotion of Prostitution – MCA 45-5-603(1)(b)
• Incest, victim under 18 years and offender three or more years older than victim – MCA 45-5-507
• Indecent Exposure
  • victim under 18 years and offender 18 years or older – MCA 45-5-504(1)
  • third or subsequent offense – MCA 45-5-504(2)(c)
• Kidnapping – MCA 45-5-302
• Sexual Abuse of Children – MCA 45-5-625
• Sexual Assault – MCA 45-5-502(3)
• Sexual Intercourse without Consent – MCA 45-5-503
• Unlawful Restraint, victim less than 18 years and offender not a parent of victim – MCA 45-5-301

Reasonably equivalent offenses of another state or the federal government also qualify.

Violent Offenses

In 1995, the Montana Legislature required certain violent offenders to register in Montana in addition
to sexual offenders. Montana is one of the few states that register violent offenders.

• Aggravated Assault – MCA 45-5-202
• Arson – MCA 45-6-103
• Assault on Minor – MCA 45-5-212
• Assault on Peace Officer or Judicial Officer – MCA 45-5-210(1)(b),
  (1)(c) or (1)(d)
• Assault with Weapon – MCA 45-5-213
• Deliberate Homicide – MCA 45-5-102
• Mitigated Deliberate Homicide – MCA 45-5-103
• Operation of Unlawful Clandestine Laboratory – MCA 45-9-132
• Partner or Family Member Assault, third or subsequent conviction – MCA 45-5-206
• Robbery – MCA 45-5-401

Reasonably equivalent offenses of another state or the federal government also qualify.

Qualifying Sexual and Violent Offenders

Offenders who have been convicted of both a qualifying sexual offense and a qualifying violent offense
are listed on the Registry as sexual and violent offenders.