What action warrants a sexual harrasment case?

It probably does not constitute sexual harassment. Sexual harassment is conduct that is directed to or affects you; however, from what you describe, you improperly looked at a personal email between two managers. The law does not regulate how people think or what they privately say to third parties; it regulates how they treat and act towards employees. If this email was not meant for or sent to you but was just private comments between two managers, then it is most likely not sexual harassment. Moreover, even if it could have been considered sexual harassment, you can only sue for compensation for actual injuries or damages (such as loss of job, lower pay, denial of promotion, or enduring a hostile workplace) you have suffered. If there is nothing more than one inappropriate email which you only became aware of by looking at your manager's computer, no harm has been done to you, and there is nothing you could sue for or recover.

Note that if you have suffered some harm that may be related to sexual harassment or discrimination--such as you are paid less then male coworkers, or were passed up for promotion--then this email could be evidence of a discriminatory mindset and could help prove that the harm you suffered was due to sexual discrimination or harassment. It's simply most likely not itself anything you could take action about.

Finally bear in mind that looking at another person's email is itself wrongful--you may have violated some laws in relation to electronic privacy, and you certainly have done something you could be lawfully fired for cause over.