There are two theories of liability regarding sexual harassment claims. One is disparate treatment and the other is disparate impact.
Disparate treatment bases the claim on whether the claimant was treated different than other employees who are similarly situated. In International Brotherhood of Teamsters v. United States, 431 U.S. 324 (1997) the United States Supreme Court held that disparate treatment occurs when "the employer simply treats some people less favorably than others because of their race, color, religion, sex or national origin."
Disparate Treatment vs. Disparate Impact Part II
Disparate impact bases the claim on whether a facially neutral company policy or other customary practice of the company has a discriminatory impact on a protected class of employees to which the claimant belongs.