Why are most people in prison male?

Answer

Because, in the past it was believed that only men could be sex offenders. It was believed that rape and such were the only crimes that were sex crimes.

Now with the sex offender registration laws we need to be aware:

A Sex Offender is any person of any age, sex, religion, or political party that for any reason has been convicted of any sex related crime; felony or a misdemeanor. These crimes do include rape they also include the following depending on where one lives; urinating in public; streaking; prostitution; oral sex; sex outside marriage; sex between consenting teenage; homosexual acts; anal sex; bestiality (even not involving animals); indecent exposure; self posting ones picture on the internet if one is seventeen years of age; etcetera.

Answer

Culturally, we tend to assume almost all sexual offenders are men because of the narrow definition most people use when talking about sex offenses: sexual assault (forcible rape, etc.) and child molestation. Part of the issue relates to male aggression (testosterone and social influences) and the power differential between men and women, making a man more likely to be physically capable of overpowering and assaulting a woman. Another possible factor involves men's reluctance to report a non-consensual sexual act.

Due to the bias against men, female sex offenders have not been studied extensively, but recent reports indicate approximately 10% of child molestation charges are against women. The rate is almost infinitesimal for female-initiated adult sexual assault.

The actions constituting a "sex crime" vary by state, but the US Supreme Court ruling in Lawrence v. Texas, (2003) should result in decriminalization or non-enforcement of many above-listed acts, as long as they are occur in private between consenting adults.