https://www.supremecourt.gov/opinions/19pdf/17-1618_hfci.pdf
Nor does it matter that, when an employer treats one employee worse because of that individual’s sex, other factors may contribute to the decision. Consider an employer with a policy of firing any woman he discovers to be a Yankees fan. Carrying out that rule because an employee is a woman and a fan of the Yankees is a firing “because of sex” if the employer would have tolerated the same allegiance in a male employee. Likewise here.

When an employer fires an employee because she is homo-sexual or transgender, two causal factors may be in play— both the individual’s sex and something else (the sex to which the individual is attracted or with which the individual identifies). But Title VII doesn’t care. If an employer would not have discharged an employee but for that individual’s sex, the statute’s causation standard is met, and liability may attach.